Halogenated methanes produced in the oceans are important as carriers of chlorine, bromine, and iodine into the atmosphere. There they play roles in the regulation of ozone in the stratosphere and perhaps in the Arctic troposphere at polar sunrise. While the mechanisms for the production of some polyhalogenated compounds by marine macrophytes have previously been substantially elucidated, the same has not been true in the case of marine phytoplankton. We describe laboratory experiments on the production of various brominated and iodinated compounds in cultures of marine diatoms, obtained from the Provasoli-Guillard Center for Culture of Marine Phytoplankton collections (Bigelow Laboratory for Ocean Sciences, Maine, USA; CCMP). Species examined included Nitzschia sp. (CCMP 580), Nitzschia arctica, Porosira glacialis, and two Navicula sp. (CCMP 545 and 546). A suite of brominated compounds, notably bromoform and dibromomethane, is produced by the Nitzschia and Porosira species. Nitzschia sp. (CCMP 580) was grown in sufficient quantities to allow the identification of a bromoperoxidase enzyme, which is assumed to be responsible not only for the CHBr3 and CH2Br2 production but also for CH2I2 which was measured in those cultures. Chloroiodomethane was produced, either directly by the algae or by a photochemical reaction of CH2I2. One Navicula species (CCMP 545), found to produce CH2I2 and CH2ClI, was shown to possess an iodoperoxidase. Bromoform and dibromomethane were not detected in cultures of this species. Other compounds produced in certain of these non axenic cultures included methyl and ethyl iodide, and bromoiodomethane. ¿ American Geophysical Union 1996