Hexachlorocyclohexanes (HCHs) are the most abundant pesticides in arctic air and water. We have calculated in situ microbial degradation rates of &agr;- and &ggr;-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 &agr;-HCH, which provide the first direct evidence of microbial degradation, allow rates to be estimated. The half-lives due to microbial activity for (+)-&agr;-HCH, (-)-&agr;-HCH and &ggr;-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, &agr;-HCH and 23 t y-1, &ggr;-HCH. These tonnages represent 29--37% of the annual HCH removal from the Arctic Ocean. ¿ 2000 American Geophysical Union