The blowout of the Macondo oil well in the Gulf of Mexico in April 2010 injected up to 500,000 tonnes of natural gas, mainly methane, into the deep sea(1). Most of the methane released was thought to have been consumed by marine microbes between July and August 2010(2,3). Here, we report spatially extensive measurements of methane concentrations and oxidation rates in the nine months following the spill. We show that although gas- rich deepwater plumes were a short- lived feature, water column concentrations of methane remained above background levels throughout the rest of the year. Rates of microbial methane oxidation peaked in the deepwater plumes in May and early June, coincident with a rapid rise in the abundance of known and new methane- oxidizing microbes. At this time, rates of methane oxidation reached up to 5,900 nmol l(-1) d(-1)-the highest rates documented in the global pelagic ocean before the blowout(4). Rates of methane oxidation fell to less than 50 nmol l(-1) d(-1) in late June, and continued to decline throughout the remainder of the year. We suggest the precipitous drop in methane consumption in late June, despite the persistence of methane in the water column, underscores the important role that physiological and environmental factors play in constraining the activity of methane- oxidizing bacteria in the Gulf of Mexico.