Pseudomonas strain Ptm(+) grew on cl-hexachlorocyclohexane (HCH, CAS no. 319846), using it as the sole source of carbon and energy. In a replacement-culture study, with the non-insecticidal alpha-HCH, gamma-HCH (CAS no. 58899) was the first metabolite noticed at 6 h, and transient accumulation of insecticidal gamma-HCH occurred for up to Is h. Although delta-(CAS no. 319868) and beta-isomers (CAS no. 319857) were also detected, their concentrations were very low. By 18 h of incubation, about 23% of the alpha-HCH added was transformed into the gamma-isomer. Subsequently, the concentration of gamma-HCH in the medium fell. Thin-layer chromatography, gas chromatography, gas chromatography-mass spectrometry and mosquito-larval bioassay analyses confirmed the formation of gamma-HCH. This was associated with the formation of three more metabolites.