when i open my script it open it in notepad rather opening it in monodevelop

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1

i m using unity 4.3.1

2

I would guess you try to open it via windows explorer. Otherwise, you need to go to Edit->Preferences->ExternalTool->ExternalScriptEditor

MonoDevelop is the one by default but VS will show if it is on your computer or you can assign any with Browse.

3

Open Unity, go to: Edit → Preferences → External Tools → External Script Editor

And see if MonoDevelop is selected.