γ-1,3,4,5,6-Pentachlorocyclohexa-1-ene (γ-PCCH), 1,2,4-trichlorobenzene (1,2,4-TCB) and 2,5-dichlorophenol (2,5-DCP) were identified as aerobic initial degradation products of γ-hexachlorocyclohexane (γ-HCH) by Pseudomonas paucimobilis UT26. The dehydrochlorination of γ-HCH to γ-PCCH was thought to occur stereoselectively at a trans and diaxial pair of hydrogen and chlorine (TD-pair) by P. paucimobilis UT26. Conformational analysis with molecular orbital calculations revealed that γ-PCCH also possesses a TD-pair of 5-position hydrogen and 6-position chlorine in its cyclohexene ring. Lineweaver-Burk plot suggested that γ-PCCH was a specific competitive inhibitor of the dehydrochlorination of γ-HCH. Considering these results, γ-PCCH was suggested to be dehydrochlorinated in a similar way to γ-HCH to yield an unidentified intermediate, 1,3,4,6-tetrachlorocyclohexa-1,4-diene (1,4-TCDN). 1,2,4-TCB and 2,5-DCP were considered to be produced from 1,4-TCDN by different reaction routes: 1,2,4-TCB was through a non-enzymatic dehydrochlorinative rearrangement of 1,4-TCDN and 2,5-DCP was through a similar rearrangement of 2,4,5-trichloro-2,5-cyclohexadiene-1-ol (2,4,5-DNOL), another unidentified intermediate from 1,4-TCDN.