A rapid transformation of eicosapentaenoic acid is initiated upon disintegration of cells of the marine diatom Stephanopyxis turris. In contrast to any other fatty acid cleaving activity known, this diatom releases a blend of chlorinated C8 hydrocarbons in addition to an unsaturated 12-oxo acid. Using labeled precursors, trapping experiments, and selective inhibition of enzyme activities, we were able to show that a lipoxygenase activates the C20 fatty acid to a hydroperoxide, which is subsequently cleaved by a new hydroperoxide halolyase to the C8 and C12 products. This adds a fundamentally new pathway to the limited set of halogenating enzymatic activities known from nature.