This study presents shipboard measurements of the loss rate constants of methyl bromide and methyl chloride in surface seawater in the Southern Ocean, using a 13C stable isotope incubation technique. The measurements were made during October--December, 2001, on a cruise track extending from Hobart, Tasmania to Buchanan Bay (Mertz Glacier) at the coast of Antarctica (46--67¿S, 138--145¿E). Significant loss rates were measured for both compounds, even in very cold waters where chemical loss rates were negligible. These observations are attributed to biological uptake, and they explain the tendency for high latitude waters to be undersaturated with respect to atmospheric methyl bromide and methyl chloride. These observations are the first open ocean measurements demonstrating the biological degradation of methyl chloride.