Methyl chloride (CH3Cl) is the most abundant halocarbon in the atmosphere, and constitutes a significant fraction of the total atmospheric halogen burden. Chemical reactions of CH3Cl in seawater are slow, and it has been believed that the oceans are not an important sink for this compound. However, direct measurements of CH3Cl degradation rates in coastal seawater (Bedford Basin, Nova Scotia), using a stable isotope incubation technique, indicate rapid loss attributed to microbial activity. A series of weekly measurements from March 2000 to May 2001 yielded degradation rates ranging from 0--30% d-1, with an annual mean of 7.4% d-1. If biological uptake of CH3Cl occurs throughout the oceans at similar rates, the mean partial atmospheric lifetime of CH3Cl with respect to oceanic removal could be a few years, rather than several decades as previously thought. This rapid removal would make the oceans a major sink for CH3Cl and lower the overall atmospheric lifetime of CH3Cl from the current estimate of 1.3 to about 1.0 years. Measurements of the degradation rate of CH3Cl in open ocean waters are needed in order to quantify the oceanic uptake rate.