The mass dependence of the star formation history of disk galaxies.

Visible (U,B,V), Near Infrared (H) and UV (λ2000A) photometric measurements of a complete sample of 928 nearby (z<0.03) late-type (spiral and irregular) galaxies are analyzed. The four sets of color indices UV-V, B-H, B-V, U-B are found tightly correlated with the galaxy H band luminosity (which is a reliable mass indicator). The synthesis population model of Bruzual & Charlot (1993) characterized by a Salpeter IMF and an exponential star formation history predict color indices consistent with the observed ones, under the assumption that galaxies of different luminosity all have similar ages (about 10GYrs). The observed correlation of color indices with H luminosity is reproduced by the model assuming that the exponential SFR decay time (T) is as short as 0.5GYrs (quasi instantaneous burst) for giant disk galaxies (LH=10^11^L_sun_) and >5GYrs (quasi constant SFR) for dwarf galaxies. The galaxy mass could be the most important factor in the regulation of the star formation history of late-type galaxies.