Natural volatile halocarbons are important as carriers of reactive halogens to the troposphere and, in the case of the more stable compounds, to the stratosphere. Bromoform (CHBr3) has been of particular interest as a potential source of bromine which might account for sudden ozone depletion events in the Arctic boundary layers. The oceans have been shown to be major contributors of volatile halocarbons to the atmosphere, but the sources of halocarbons within them have been unknown except for macrophytic algae which are normally confined to the coastal zone. Here we report experiments that demonstrate that certain unialgal cultures of marine phytoplankton produce a suite of halocarbons (CHBr3, CHBr2Cl, CH2Br2). The production rate of each of these halocarbons is dependent on both species and growth stage. Chloroiodomethane which also appeared in the cultures could be attributable to photochemical production from a precursor (CH2I2). Great caution should be shown in extrapolating the rates estimated from these controlled experiments to the marine environment.