A system for user-guided audio source separation is presented in this article. Following previous works on time-frequency music representations, the proposed User Interface allows the user to select the desired audio source, by means of the assumed fundamental frequency (F0) track of that source. The system then automatically refines the selected F0 tracks, estimates and separates the corresponding source from the mixture. The interface was tested and the separation results compare positively to the results of a fully automatic system, showing that the F0 track selection improves the separation performance.