Thank you for the reply.
Sorry but I think that I wrote wrong my question (I'm Belgian :s)
I know that I²C is a bus protocol, and I use it to communicate between my 2 arduino boards.
The communication works great, and now I want to plug the screen to display results. The screen take all the Input/Output pins of the Arduino UNO so my question is : can I use pin A4 and A5 of the Arduino on which the screen is plugged for I²C communication or are these pins used by the screen?