We investigate two types of nonlinear interaction between collinear femtosecond laser pulses with power slightly above the critical for self-focusing P cr . In the first case we study energy exchange between filaments. The model describes this process through a degenerate fourphoton parametric mixing (FPPM) scheme and requests initial phase difference between the waves. When there is no initial phase difference between the pulses, the FPPM process does not work. In this case the second type of interaction is obtained as merging between two, three or four filaments in a single filament with higher power. It is found that in the second case the interflow between the filaments has the potential for interaction due to cross-phase modulation (CPM).