diff --git a/src/RWStl/RWStl_Reader.cxx b/src/RWStl/RWStl_Reader.cxx
index e8514712db..b29bd3e010 100644
--- a/src/RWStl/RWStl_Reader.cxx
+++ b/src/RWStl/RWStl_Reader.cxx
@@ -149,7 +149,7 @@ Standard_Boolean RWStl_Reader::Read (const char* theFile,
   // (80 bytes header + 4 bytes facet count + 50 bytes for one facet);
   // thus assume files shorter than 134 as Ascii without probing
   // (probing may bring stream to fail state if EOF is reached)
-  bool isAscii = ((size_t)theEnd < THE_STL_MIN_FILE_SIZE || IsAscii (aStream));
+  bool isAscii = ((size_t)theEnd < THE_STL_MIN_FILE_SIZE || IsAscii (aStream, true));
 
   Standard_ReadLineBuffer aBuffer (THE_BUFFER_SIZE);
 
@@ -185,7 +185,8 @@ Standard_Boolean RWStl_Reader::Read (const char* theFile,
 //purpose  :
 //==============================================================================
 
-Standard_Boolean RWStl_Reader::IsAscii (Standard_IStream& theStream)
+Standard_Boolean RWStl_Reader::IsAscii (Standard_IStream& theStream,
+                                        const bool        isSeekgAvailable)
 {
   // read first 134 bytes to detect file format
   char aBuffer[THE_STL_MIN_FILE_SIZE];
@@ -196,10 +197,18 @@ Standard_Boolean RWStl_Reader::IsAscii (Standard_IStream& theStream)
     return true;
   }
 
-  // put back the read symbols
-  for (std::streamsize aByteIter = aNbRead; aByteIter > 0; --aByteIter)
+  if (isSeekgAvailable)
   {
-    theStream.unget();
+    // get back to the beginning
+    theStream.seekg(0, theStream.beg);
+  }
+  else
+  {
+    // put back the read symbols
+    for (std::streamsize aByteIter = aNbRead; aByteIter > 0; --aByteIter)
+    {
+      theStream.unget();
+    }
   }
 
   // if file is shorter than size of binary file with 1 facet, it must be ascii
diff --git a/src/RWStl/RWStl_Reader.hxx b/src/RWStl/RWStl_Reader.hxx
index f1b2f2309b..671f79d66f 100644
--- a/src/RWStl/RWStl_Reader.hxx
+++ b/src/RWStl/RWStl_Reader.hxx
@@ -45,9 +45,12 @@ public:
                                          const Message_ProgressRange& theProgress);
 
   //! Guess whether the stream is an Ascii STL file, by analysis of the first bytes (~200).
-  //! The function attempts to put back the read symbols to the stream which thus must support ungetc().
+  //! If the stream does not support seekg() then the parameter isSeekgAvailable should
+  //! be passed as 'false', in this case the function attempts to put back the read symbols
+  //! to the stream which thus must support ungetc().
   //! Returns true if the stream seems to contain Ascii STL.
-  Standard_EXPORT Standard_Boolean IsAscii (Standard_IStream& theStream);
+  Standard_EXPORT Standard_Boolean IsAscii (Standard_IStream& theStream,
+                                            const bool isSeekgAvailable);
 
   //! Reads STL data from binary stream.
   //! The stream must be opened in binary mode.