Metarhizium anisopliae has potential as a biological control agent. Included among its hosts are certain insect pests of brassica crops. Brassica species produce isothiocyanates, some of which are known to be fungitoxic. In our study, isothiocyanates inhibited both germination and subsequent growth by M. anisopliae in vitro and its ability to infect insects. Conidia were more sensitive than the mycelium to these compounds, the most fungistatic of which were phenylethyl-, 2-chlorophenyl- and allyl-isothiocyanates. Appressorium production in vitro was suppressed by all isothiocyanates except allyl- and propyl-isothiocyanates, which appeared to stimulate appressorium formation. Phenylethyl- and 3-butenyl isothiocyanates, which are present in several of the plant hosts of Phaedon cochleariae, reduced the pathogenicity of M. anisopliae when inoculated insects were exposed to their vapours. These findings have implications for the efficacy of biocontrol of brassica pests by this fungus.