diff --git a/flang/runtime/io-api.cpp b/flang/runtime/io-api.cpp
--- a/flang/runtime/io-api.cpp
+++ b/flang/runtime/io-api.cpp
@@ -111,8 +111,8 @@
   if (unitNumber == DefaultUnit) {
     unitNumber = DIR == Direction::Input ? 5 : 6;
   }
-  ExternalFileUnit &unit{
-      ExternalFileUnit::LookUpOrCrash(unitNumber, terminator)};
+  ExternalFileUnit &unit{ExternalFileUnit::LookUpOrCreateAnonymous(
+      unitNumber, DIR, false /*formatted*/, terminator)};
   if (unit.access == Access::Direct) {
     terminator.Crash("List-directed I/O attempted on direct access file");
     return nullptr;
@@ -150,8 +150,8 @@
   if (unitNumber == DefaultUnit) {
     unitNumber = DIR == Direction::Input ? 5 : 6;
   }
-  ExternalFileUnit &unit{
-      ExternalFileUnit::LookUpOrCrash(unitNumber, terminator)};
+  ExternalFileUnit &unit{ExternalFileUnit::LookUpOrCreateAnonymous(
+      unitNumber, DIR, false /*formatted*/, terminator)};
   if (unit.isUnformatted) {
     terminator.Crash("Formatted I/O attempted on unformatted file");
     return nullptr;
@@ -185,8 +185,8 @@
 Cookie BeginUnformattedIO(
     ExternalUnit unitNumber, const char *sourceFile, int sourceLine) {
   Terminator terminator{sourceFile, sourceLine};
-  ExternalFileUnit &unit{
-      ExternalFileUnit::LookUpOrCrash(unitNumber, terminator)};
+  ExternalFileUnit &unit{ExternalFileUnit::LookUpOrCreateAnonymous(
+      unitNumber, DIR, true /*unformatted*/, terminator)};
   if (!unit.isUnformatted) {
     terminator.Crash("Unformatted output attempted on formatted file");
   }
@@ -223,7 +223,7 @@
   bool wasExtant{false};
   Terminator terminator{sourceFile, sourceLine};
   ExternalFileUnit &unit{
-      ExternalFileUnit::LookUpOrCreate(unitNumber, terminator, &wasExtant)};
+      ExternalFileUnit::LookUpOrCreate(unitNumber, terminator, wasExtant)};
   return &unit.BeginIoStatement<OpenStatementState>(
       unit, wasExtant, sourceFile, sourceLine);
 }
@@ -231,10 +231,11 @@
 Cookie IONAME(BeginOpenNewUnit)( // OPEN(NEWUNIT=j)
     const char *sourceFile, int sourceLine) {
   Terminator terminator{sourceFile, sourceLine};
+  bool ignored{false};
   ExternalFileUnit &unit{ExternalFileUnit::LookUpOrCreate(
-      ExternalFileUnit::NewUnit(terminator), terminator)};
+      ExternalFileUnit::NewUnit(terminator), terminator, ignored)};
   return &unit.BeginIoStatement<OpenStatementState>(
-      unit, false /*wasExtant*/, sourceFile, sourceLine);
+      unit, false /*was an existing file*/, sourceFile, sourceLine);
 }
 
 Cookie IONAME(BeginClose)(
diff --git a/flang/runtime/unit-map.h b/flang/runtime/unit-map.h
--- a/flang/runtime/unit-map.h
+++ b/flang/runtime/unit-map.h
@@ -27,16 +27,11 @@
   }
 
   ExternalFileUnit &LookUpOrCreate(
-      int n, const Terminator &terminator, bool *wasExtant) {
+      int n, const Terminator &terminator, bool &wasExtant) {
     CriticalSection critical{lock_};
     auto *p{Find(n)};
-    if (wasExtant) {
-      *wasExtant = p != nullptr;
-    }
-    if (p) {
-      return *p;
-    }
-    return Create(n, terminator);
+    wasExtant = p != nullptr;
+    return p ? *p : Create(n, terminator);
   }
 
   ExternalFileUnit &NewUnit(const Terminator &terminator) {
diff --git a/flang/runtime/unit.h b/flang/runtime/unit.h
--- a/flang/runtime/unit.h
+++ b/flang/runtime/unit.h
@@ -39,7 +39,10 @@
   static ExternalFileUnit *LookUp(int unit);
   static ExternalFileUnit &LookUpOrCrash(int unit, const Terminator &);
   static ExternalFileUnit &LookUpOrCreate(
-      int unit, const Terminator &, bool *wasExtant = nullptr);
+      int unit, const Terminator &, bool &wasExtant);
+  static ExternalFileUnit &LookUpOrCreateAnonymous(
+      int unit, Direction, bool isUnformatted, const Terminator &);
+  static ExternalFileUnit &CreateNew(int unit, const Terminator &);
   static ExternalFileUnit *LookUpForClose(int unit);
   static int NewUnit(const Terminator &);
   static void CloseAll(IoErrorHandler &);
diff --git a/flang/runtime/unit.cpp b/flang/runtime/unit.cpp
--- a/flang/runtime/unit.cpp
+++ b/flang/runtime/unit.cpp
@@ -10,6 +10,7 @@
 #include "io-error.h"
 #include "lock.h"
 #include "unit-map.h"
+#include <cstdio>
 
 namespace Fortran::runtime::io {
 
@@ -46,10 +47,38 @@
 }
 
 ExternalFileUnit &ExternalFileUnit::LookUpOrCreate(
-    int unit, const Terminator &terminator, bool *wasExtant) {
+    int unit, const Terminator &terminator, bool &wasExtant) {
   return GetUnitMap().LookUpOrCreate(unit, terminator, wasExtant);
 }
 
+ExternalFileUnit &ExternalFileUnit::LookUpOrCreateAnonymous(
+    int unit, Direction dir, bool isUnformatted, const Terminator &terminator) {
+  bool exists{false};
+  ExternalFileUnit &result{
+      GetUnitMap().LookUpOrCreate(unit, terminator, exists)};
+  if (!exists) {
+    // I/O to an unconnected unit reads/creates a local file, e.g. fort.7
+    std::size_t pathMaxLen{32};
+    auto path{SizedNew<char>{terminator}(pathMaxLen)};
+    std::snprintf(path.get(), pathMaxLen, "fort.%d", unit);
+    IoErrorHandler handler{terminator};
+    result.OpenUnit(
+        dir == Direction::Input ? OpenStatus::Old : OpenStatus::Replace,
+        Position::Rewind, std::move(path), std::strlen(path.get()), handler);
+    result.isUnformatted = isUnformatted;
+  }
+  return result;
+}
+
+ExternalFileUnit &ExternalFileUnit::CreateNew(
+    int unit, const Terminator &terminator) {
+  bool wasExtant{false};
+  ExternalFileUnit &result{
+      GetUnitMap().LookUpOrCreate(unit, terminator, wasExtant)};
+  RUNTIME_CHECK(terminator, !wasExtant);
+  return result;
+}
+
 ExternalFileUnit *ExternalFileUnit::LookUpForClose(int unit) {
   return GetUnitMap().LookUpForClose(unit);
 }
@@ -155,14 +184,14 @@
   Terminator terminator{__FILE__, __LINE__};
   IoErrorHandler handler{terminator};
   unitMap = New<UnitMap>{terminator}().release();
-  ExternalFileUnit &out{ExternalFileUnit::LookUpOrCreate(6, terminator)};
+  ExternalFileUnit &out{ExternalFileUnit::CreateNew(6, terminator)};
   out.Predefine(1);
   out.set_mayRead(false);
   out.set_mayWrite(true);
   out.set_mayPosition(false);
   out.SetDirection(Direction::Output, handler);
   defaultOutput = &out;
-  ExternalFileUnit &in{ExternalFileUnit::LookUpOrCreate(5, terminator)};
+  ExternalFileUnit &in{ExternalFileUnit::CreateNew(5, terminator)};
   in.Predefine(0);
   in.set_mayRead(true);
   in.set_mayWrite(false);