Index: test/test_file.rb
===================================================================
--- test/test_file.rb	(revision 4772)
+++ test/test_file.rb	(working copy)
@@ -274,7 +274,10 @@
     File.delete(filename)
 
     assert_raises(IOError) { File.new(filename, File::CREAT) << 'b' }
+
     File.delete(filename)
+
+    assert_raises(Errno::EISDIR) { File.new(".", "w") }
   end
 
   # http://jira.codehaus.org/browse/JRUBY-1231
Index: src/org/jruby/RubyFile.java
===================================================================
--- src/org/jruby/RubyFile.java	(revision 4772)
+++ src/org/jruby/RubyFile.java	(working copy)
@@ -51,6 +51,7 @@
 import org.jruby.runtime.ThreadContext;
 import org.jruby.runtime.builtin.IRubyObject;
 import org.jruby.util.ByteList;
+import org.jruby.util.DirectoryAsFileException;
 import org.jruby.util.Chmod;
 import org.jruby.util.IOHandler;
 import org.jruby.util.IOHandlerNull;
@@ -278,6 +279,8 @@
             registerIOHandler(handler);
         } catch (InvalidValueException e) {
         	throw getRuntime().newErrnoEINVALError();
+        } catch (DirectoryAsFileException e) {
+        	throw getRuntime().newErrnoEISDirError();
         } catch (FileNotFoundException e) {
         	throw getRuntime().newErrnoENOENTError();
         } catch (IOException e) {
Index: src/org/jruby/Ruby.java
===================================================================
--- src/org/jruby/Ruby.java	(revision 4772)
+++ src/org/jruby/Ruby.java	(working copy)
@@ -1605,6 +1605,10 @@
         return newRaiseException(getModule("Errno").getClass("ENOENT"), "File not found");
     }
 
+    public RaiseException newErrnoEISDirError() {
+        return newRaiseException(getModule("Errno").getClass("EISDIR"), "Is a directory");
+    }
+
     public RaiseException newErrnoESPIPEError() {
         return newRaiseException(getModule("Errno").getClass("ESPIPE"), "Illegal seek");
     }
Index: src/org/jruby/util/IOHandlerSeekable.java
===================================================================
--- src/org/jruby/util/IOHandlerSeekable.java	(revision 4772)
+++ src/org/jruby/util/IOHandlerSeekable.java	(working copy)
@@ -71,6 +71,8 @@
         this.cwd = runtime.getCurrentDirectory();
         JRubyFile theFile = JRubyFile.create(cwd,path);
         
+        if (theFile.isDirectory() && modes.isWritable()) throw new DirectoryAsFileException();
+        
         if (modes.isCreate()) theFile.createNewFile();
         
         if(!theFile.exists()) {
Index: src/org/jruby/util/DirectoryAsFileException.java
===================================================================
--- src/org/jruby/util/DirectoryAsFileException.java	(revision 0)
+++ src/org/jruby/util/DirectoryAsFileException.java	(revision 0)
@@ -0,0 +1,34 @@
+/*
+ ***** BEGIN LICENSE BLOCK *****
+ * Version: CPL 1.0/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Common Public
+ * License Version 1.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of
+ * the License at http://www.eclipse.org/legal/cpl-v10.html
+ *
+ * Software distributed under the License is distributed on an "AS
+ * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * rights and limitations under the License.
+ *
+ * Copyright (C) 2007 Thomas E Enebo <enebo@acm.org>
+ * 
+ * Alternatively, the contents of this file may be used under the terms of
+ * either of the GNU General Public License Version 2 or later (the "GPL"),
+ * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the CPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the CPL, the GPL or the LGPL.
+ ***** END LICENSE BLOCK *****/
+package org.jruby.util;
+
+import java.io.IOException;
+
+public class DirectoryAsFileException extends IOException {
+}