Cell death and mutagenesis in bleomycin-treated cells of Thiocapsa roseopersicina (a purple sulfur bacterium) was studied by cultivation in a semisolid medium (agar-shake technique). This technique has also proven useful in assessing the frequency of antibiotic mutations by detecting and counting individual colonies of Thiocapsa roseopersicina. The frequencies of spontaneous mutants resistant to ampicillin, rifampicin, cloramphenicol, tetracycline, kanamycin, streptomycin, and neomycin were also studied: they ranged between 2×10-9 and 9×10-8. Bleomycin (4 μg/ml) sharply increased the frequency of ampicillin-resistant mutants, from 10-8 (spontaneous) to 4×10-4 (induced), in 17 h. An inducible, error-prone mechanisms of DNA synthesis seems to be responsible for this enhancement of the mutagenic effect. This is the first report on the sensitivity to several antibiotics, and capacity of lethality and mutagenesis by bleomycin has been studied in a purple sulfur bacterium. © 1995 Springer-Verlag.