Sub test()
Dim i As Long, monClasseur As String
monClasseur = "toto.xlsm"
For i = 1 To Workbooks.Count
If LCase(Workbooks(i).Name) = LCase(monClasseur) Then Exit For
Next i
If i = Workbooks.Count + 1 Then MsgBox monClasseur & " n'est pas ouvert." Else MsgBox monClasseur & " => Index = " & i
End Sub