diff -r eccfb9b597c1 Lib/idlelib/PyShell.py
--- a/Lib/idlelib/PyShell.py	Wed Jun 08 17:55:49 2016 -0400
+++ b/Lib/idlelib/PyShell.py	Thu Jun 09 18:36:50 2016 -0400
@@ -1396,6 +1396,17 @@
         self.shell.close()
 
 
+def fix_x11_paste(root):
+    "Make paste replace selection on x11.  See issue #5124."
+    if root._windowingsystem == 'x11':
+        for cls in 'Text', 'Entry', 'Spinbox':
+            root.bind_class(
+                cls,
+                '<<Paste>>',
+                'catch {%W delete sel.first sel.last}\n' +
+                        root.bind_class(cls, '<<Paste>>'))
+
+
 usage_msg = """\
 
 USAGE: idle  [-deins] [-t title] [file]*
@@ -1528,8 +1539,10 @@
                                     'editor-on-startup', type='bool')
     enable_edit = enable_edit or edit_start
     enable_shell = enable_shell or not enable_edit
+
     # start editor and/or shell windows:
     root = Tk(className="Idle")
+    root.withdraw()
 
     # set application icon
     icondir = os.path.join(os.path.dirname(__file__), 'Icons')
@@ -1544,7 +1557,7 @@
         root.wm_iconphoto(True, *icons)
 
     fixwordbreaks(root)
-    root.withdraw()
+    fix_x11_paste(root)
     flist = PyShellFileList(root)
     macosxSupport.setupApp(root, flist)
 
diff -r eccfb9b597c1 Lib/idlelib/idle_test/test_editmenu.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/idlelib/idle_test/test_editmenu.py	Thu Jun 09 18:36:50 2016 -0400
@@ -0,0 +1,71 @@
+'''Test (selected) IDLE Edit menu items.
+
+Edit modules have their own test files files
+'''
+from test.support import requires
+requires('gui')
+import tkinter as tk
+import unittest
+from idlelib import PyShell
+
+class PasteTest(unittest.TestCase):
+    '''Test pasting into widgets that allow pasting.
+
+    On X11, replacing selections requires tk fix.
+    '''
+    @classmethod
+    def setUpClass(cls):
+        cls.root = root = tk.Tk()
+        PyShell.fix_x11_paste(root)
+        cls.text = tk.Text(root)
+        cls.entry = tk.Entry(root)
+        cls.spin = tk.Spinbox(root)
+        root.clipboard_clear()
+        root.clipboard_append('two')
+
+    @classmethod
+    def tearDownClass(cls):
+        del cls.text, cls.entry, cls.spin
+        cls.root.clipboard_clear()
+        cls.root.update_idletasks()
+        cls.root.destroy()
+        del cls.root
+
+    def test_paste_text(self):
+        "Test pasting into text with and without a selection."
+        text = self.text
+        for tag, ans in ('', 'onetwo\n'), ('sel', 'two\n'):
+            with self.subTest(tag=tag, ans=ans):
+                text.delete('1.0', 'end')
+                text.insert('1.0', 'one', tag)
+                text.event_generate('<<Paste>>')
+                self.assertEqual(text.get('1.0', 'end'), ans)
+
+    def test_paste_entry(self):
+        "Test pasting into an entry with and without a selection."
+        # On 3.6, generated <<Paste>> fails without empty select range
+        # for 'no selection'.  Live widget works fine.
+        entry = self.entry
+        for end, ans in (0, 'onetwo'), ('end', 'two'):
+            with self.subTest(entry=entry, end=end, ans=ans):
+                entry.delete(0, 'end')
+                entry.insert(0, 'one')
+                entry.select_range(0, end)  # see note
+                entry.event_generate('<<Paste>>')
+                self.assertEqual(entry.get(), ans)
+
+    def test_paste_spin(self):
+        "Test pasting into a spinbox with and without a selection."
+        # See note above for entry.
+        spin = self.spin
+        for end, ans in (0, 'onetwo'), ('end', 'two'):
+            with self.subTest(end=end, ans=ans):
+                spin.delete(0, 'end')
+                spin.insert(0, 'one')
+                spin.selection('range', 0, end)  # see note
+                spin.event_generate('<<Paste>>')
+                self.assertEqual(spin.get(), ans)
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)