While release of isoprene from the terrestrial biosphere is well established and known to influence the tropospheric concentrations of a number of reactive species including ozone, very little is known of its production in the ocean and possible release to the marine boundary layer. Measurements reported here of low molecular weight trace gases produced by a series of laboratory phytoplankton have shown that isoprene is a major component. Though the results confirm that the oceans are a potential source of isoprene to the atmosphere, it is not possible to extrapolate release rates from laboratory studies to the very different conditions under which marine plants grow naturally. ¿ American Geophysical Union 1994