The interaction of two femtosecond filaments has been studied in single-crystal sapphire at small crossing angles 1.2° ÷5°. Regular multifilament pattern (MFP) can be formed in the overlapping area of two beams, which depends on the pulse energy, crossing angle, and phase difference. As a special realization of MFP, formation of a single filament from two different pump beams was for the first time observed. The number of filaments in the MFP depends on the number of the interference maxima in overlapping area, whose power exceeds critical power for self-focusing. We demonstrate control of multifilament pattern, changing the phase shift between the interacting beams. Attraction and repulsion of the filaments is observed in vicinity of the overlapping region for both in- and counter-phase beams depending on the position Z along the propagation axis. Transformation of near- and far-field images of the single filament, formed by two pump beams, and spectra of its axial emission have been studied depending on the filament length. It has been shown that four-wave mixing plays a major role in forming the axial emission spectrum.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.