The effect of exposure of bacterial suspensions to UV radiation by means of the dose-response curves was assessed. The D-37 and D-10 values were used for subsequent statistical analysis of the results. The aim of this article is to evaluate the sensitivity to UV radiation of several microorganisms of different habitats (Rhizobium meliloti, Rhodobacter sphaeroides, Escherichia coli, and Deinococcus radiodurans), two mutants with nonfunctional SOS DNA repair system (R. meliloti recA(-) and E. coli recA(-)), and a mutant in the synthesis of carotenoids (R. sphaeroides crtD). The results reveal that D. radiodurans was an extremely resistant bacterium, R. meliloti was more resistant than R. sphaeroides, and E. coli was the most sensitive bacterium tested. The high sensitivity of recA(-) mutants was also verify. Moreover, it seems that the possession of pigments had no important effect in the sensitivity of R. sphaeroides to UV radiation.