The production of volatile low-molecular-weight halocarbons by macroalgae was investigated at a site on the west coast of Scotland. Levels of CH31, CH,Br,, CHBr,, CHBr,Cl, CHBrCl,, and CHC13 were elevated in beds of Laminaria digitata. Storage tests on 11 common species of brown, green, and red seaweeds indicated release of some or all of these compounds. Estimates of production rates were obtained from laboratory incubations and, where comparative data are available, are in the range of those previously reported. Measured rates of halomethane release varied with algal species and were inconsistent with halomethane formation by the release of HOBr to seawater and subsequent reaction with organic matter. The production of CHCl, was best explained by the presence of a chloroperoxidase enzyme. Laboratory experiments suggested that release rates can bc influenced by partial desiccation, light availability, tissue age, tissue wounding, and grazing. The release of CH31, CH,Br,, CHBr,, CHBr,Cl, and probably CHCl, by algae from within a rock pool was observed for the first time. Rates of in situ production were in broad agreement with laboratory studies. No production of Ccl, or of CHClCCl,, CCl,CCl,, or CH,CCl, was detected. In addition to CHBr,, macroalgae may be a significant source of CHCl, to the atmosphere.