ABSTRACT Hexachlorocyclohexanes (HCHs) are the most abundant pesticides in arctic air and water. We have calculated in situ microbial degradation rates of α- and γ-HCH for the Arctic Ocean using enantiomer ratio (ER) data and concentrations in water at the surface and after it has subducted to depth. The ERs of α-HCH, which provide the first direct evidence of microbial degradation, allow rates to be estimated. The half-lives due to microbial activity for (+)-α-HCH, (−)-α-HCH and γ-HCH (respectively 5.9 ± 1.2, 22.8 ± 4.7 and 18.8 ± 10.1 years) are 3–10 times faster than removal by the only other known breakdown mechanism, hydrolysis (Harner et al., 1999). Microbial degradation is highlighted as an important elimination process in a revised HCH loss budget for the Arctic Ocean, removing 204 t y−1, α-HCH and 23 t y−1, γ-HCH. These tonnages represent 29–37% of the annual HCH removal from the Arctic Ocean.