diff --git a/flang/include/flang/Optimizer/Dialect/FIRAttr.h b/flang/include/flang/Optimizer/Dialect/FIRAttr.h
--- a/flang/include/flang/Optimizer/Dialect/FIRAttr.h
+++ b/flang/include/flang/Optimizer/Dialect/FIRAttr.h
@@ -38,7 +38,7 @@
   using Base::Base;
   using ValueType = mlir::Type;
 
-  static constexpr llvm::StringRef getAttrName() { return "instance"; }
+  static constexpr llvm::StringRef getAttrName() { return "type_is"; }
   static ExactTypeAttr get(mlir::Type value);
 
   mlir::Type getType() const;
@@ -51,7 +51,7 @@
   using Base::Base;
   using ValueType = mlir::Type;
 
-  static constexpr llvm::StringRef getAttrName() { return "subsumed"; }
+  static constexpr llvm::StringRef getAttrName() { return "class_is"; }
   static SubclassAttr get(mlir::Type value);
 
   mlir::Type getType() const;
diff --git a/flang/include/flang/Optimizer/Dialect/FIROps.td b/flang/include/flang/Optimizer/Dialect/FIROps.td
--- a/flang/include/flang/Optimizer/Dialect/FIROps.td
+++ b/flang/include/flang/Optimizer/Dialect/FIROps.td
@@ -651,10 +651,10 @@
 
     ```mlir
       fir.select_type %arg : !fir.box<()> [
-          #fir.instance<!fir.type<type1>>, ^bb1(%0 : i32),
-          #fir.instance<!fir.type<type2>>, ^bb2(%2 : i32),
-          #fir.subsumed<!fir.type<type3>>, ^bb3(%2 : i32),
-          #fir.instance<!fir.type<type4>>, ^bb4(%1,%3 : i32,f32),
+          #fir.type_is<!fir.type<type1>>, ^bb1(%0 : i32),
+          #fir.type_is<!fir.type<type2>>, ^bb2(%2 : i32),
+          #fir.class_is<!fir.type<type3>>, ^bb3(%2 : i32),
+          #fir.type_is<!fir.type<type4>>, ^bb4(%1,%3 : i32,f32),
           unit, ^bb5]
     ```
   }];
diff --git a/flang/lib/Optimizer/Dialect/FIROps.cpp b/flang/lib/Optimizer/Dialect/FIROps.cpp
--- a/flang/lib/Optimizer/Dialect/FIROps.cpp
+++ b/flang/lib/Optimizer/Dialect/FIROps.cpp
@@ -2970,8 +2970,11 @@
 }
 
 mlir::LogicalResult fir::SelectTypeOp::verify() {
-  if (!(getSelector().getType().isa<fir::BoxType>()))
-    return emitOpError("must be a boxed type");
+  if (!(getSelector().getType().isa<fir::BaseBoxType>()))
+    return emitOpError("must be a fir.class or fir.box type");
+  if (auto boxType = getSelector().getType().dyn_cast<fir::BoxType>())
+    if (!boxType.getEleTy().isa<mlir::NoneType>())
+      return emitOpError("selector must be polymorphic");
   auto cases =
       getOperation()->getAttrOfType<mlir::ArrayAttr>(getCasesAttr()).getValue();
   auto count = getNumDest();
diff --git a/flang/test/Fir/convert-to-llvm-invalid.fir b/flang/test/Fir/convert-to-llvm-invalid.fir
--- a/flang/test/Fir/convert-to-llvm-invalid.fir
+++ b/flang/test/Fir/convert-to-llvm-invalid.fir
@@ -71,14 +71,14 @@
 // Test `fir.select_type` conversion to llvm.
 // Should have been converted.
 
-func.func @bar_select_type(%arg : !fir.box<!fir.ref<f32>>) -> i32 {
+func.func @bar_select_type(%arg : !fir.class<!fir.type<derivedst{a:f32}>>) -> i32 {
   %0 = arith.constant 1 : i32
   %2 = arith.constant 3 : i32
   // expected-error@+2{{fir.select_type should have already been converted}}
   // expected-error@+1{{failed to legalize operation 'fir.select_type'}}
-  fir.select_type %arg : !fir.box<!fir.ref<f32>> [
-    #fir.instance<!fir.int<4>>,^bb1(%0:i32),
-    #fir.instance<!fir.int<8>>,^bb2(%2:i32),
+  fir.select_type %arg : !fir.class<!fir.type<derivedst{a:f32}>> [
+    #fir.type_is<!fir.int<4>>,^bb1(%0:i32),
+    #fir.type_is<!fir.int<8>>,^bb2(%2:i32),
     unit,^bb5 ]
 ^bb1(%a : i32) :
   return %a : i32
diff --git a/flang/test/Fir/fir-ops.fir b/flang/test/Fir/fir-ops.fir
--- a/flang/test/Fir/fir-ops.fir
+++ b/flang/test/Fir/fir-ops.fir
@@ -322,8 +322,8 @@
 }
 
 // CHECK-LABEL: func @bar_select_type(
-// CHECK-SAME: [[VAL_101:%.*]]: !fir.box<!fir.type<name(param1:i32){fld:!fir.char<1>}>>) -> i32 {
-func.func @bar_select_type(%arg : !fir.box<!fir.type<name(param1:i32){fld:!fir.char<1>}>>) -> i32 {
+// CHECK-SAME: [[VAL_101:%.*]]: !fir.class<!fir.type<name(param1:i32){fld:!fir.char<1>}>>) -> i32 {
+func.func @bar_select_type(%arg : !fir.class<!fir.type<name(param1:i32){fld:!fir.char<1>}>>) -> i32 {
 
 // CHECK: [[VAL_102:%.*]] = arith.constant 1 : i32
 // CHECK: [[VAL_103:%.*]] = arith.constant 2 : i32
@@ -334,8 +334,8 @@
   %2 = arith.constant 3 : i32
   %3 = arith.constant 4 : i32
 
-// CHECK: fir.select_type [[VAL_101]] : !fir.box<!fir.type<name(param1:i32){fld:!fir.char<1>}>> [#fir.instance<!fir.int<4>>, ^bb1([[VAL_102]] : i32), #fir.instance<!fir.int<8>>, ^bb2([[VAL_104]] : i32), #fir.subsumed<!fir.int<2>>, ^bb3([[VAL_104]] : i32), #fir.instance<!fir.int<1>>, ^bb4([[VAL_103]] : i32), unit, ^bb5]
-  fir.select_type %arg : !fir.box<!fir.type<name(param1:i32){fld:!fir.char<1>}>> [ #fir.instance<!fir.int<4>>,^bb1(%0:i32), #fir.instance<!fir.int<8>>,^bb2(%2:i32), #fir.subsumed<!fir.int<2>>,^bb3(%2:i32), #fir.instance<!fir.int<1>>,^bb4(%1:i32), unit,^bb5 ]
+// CHECK: fir.select_type [[VAL_101]] : !fir.class<!fir.type<name(param1:i32){fld:!fir.char<1>}>> [#fir.type_is<!fir.int<4>>, ^bb1([[VAL_102]] : i32), #fir.type_is<!fir.int<8>>, ^bb2([[VAL_104]] : i32), #fir.class_is<!fir.int<2>>, ^bb3([[VAL_104]] : i32), #fir.type_is<!fir.int<1>>, ^bb4([[VAL_103]] : i32), unit, ^bb5]
+  fir.select_type %arg : !fir.class<!fir.type<name(param1:i32){fld:!fir.char<1>}>> [ #fir.type_is<!fir.int<4>>,^bb1(%0:i32), #fir.type_is<!fir.int<8>>,^bb2(%2:i32), #fir.class_is<!fir.int<2>>,^bb3(%2:i32), #fir.type_is<!fir.int<1>>,^bb4(%1:i32), unit,^bb5 ]
 
 // CHECK: ^bb1([[VAL_106:%.*]]: i32):
 // CHECK: return [[VAL_106]] : i32
diff --git a/flang/test/Fir/invalid.fir b/flang/test/Fir/invalid.fir
--- a/flang/test/Fir/invalid.fir
+++ b/flang/test/Fir/invalid.fir
@@ -928,3 +928,22 @@
   %0 = fir.declare %arg0(%shape) {uniq_name = "x"} : (!fir.ref<!fir.class<!fir.ptr<!fir.array<?x?xnone>>>>, !fir.shift<2>) -> !fir.ref<!fir.class<!fir.ptr<!fir.array<?x?xnone>>>>
   return
 }
+
+// -----
+
+func.func @invalid_selector(%arg : !fir.box<!fir.ref<i32>>) -> i32 {
+  %0 = arith.constant 1 : i32
+  %2 = arith.constant 3 : i32
+  // expected-error@+1{{'fir.select_type' op selector must be polymorphic}}
+  fir.select_type %arg : !fir.box<!fir.ref<i32>> [
+    #fir.type_is<!fir.int<4>>,^bb1(%0:i32),
+    #fir.type_is<!fir.int<8>>,^bb2(%2:i32),
+    unit,^bb5 ]
+^bb1(%a : i32) :
+  return %a : i32
+^bb2(%b : i32) :
+  return %b : i32
+^bb5 :
+  %zero = arith.constant 0 : i32
+  return %zero : i32
+}