ABSTRACTWhen culture blocks were incubated on V-8 agar plates containing 300 ppm streptomycin sulfate, mutants appeared as fast-growing sectors within 30 days. Streptomycin-resistant mutants were isolated from the sectors of both A1and A2mating types ofP. parasitica, P. palmivora, P. capsiciandP. cinnamomi.Streptomycin also caused mating type change inP. parasitica.Streptomycin-dependent mutants were also isolated from the fast-growing sectors of A2isolates ofP. parasiticaand A1and A2isolates ofP. palmivora, P. capsiciandP. cinnamomi.Presence of CaCl2in the medium induced a greater need for streptomycin for the dependent isolates ofP. parasitica.Fast-growing sectors also appeared when culture blocks were incubated on medium containing 100 ppm chloramphenicol. Chloramphenicol-resistant mutants were isolated from sectors of A1and A2mating type ofP. parasitica, P. palmivora, P. capsiciandP. cinnamomi.By incubating streptomycin-resistant mutants ofP. parasiticaon medium containing chloramphenicol and vice versa, mutants resistant to both antibiotics were obtained.