The susceptibility of mustard beetle, Phaedon cochleariae, larvae to the insect pathogenic fungus Metarhizium anisopliae, was influenced by the topography of the host plant, the dosage and the larval instar. At high doses (>109 conidia ml−1) more insects were likely to acquire conidia. Most of the inoculum adhered to the mid-ventral region of the abdomen. Inoculum applied to the leaf became diluted during leaf expansion and this decreased beetle mortality. Leaf expansion was slower at 10°C than at alternating temperatures of 23/10° or when constant at 23°. Slightly more larvae acquired conidia when fed on oilseed rape than Chinese cabbage or turnip. Most conidia adhered to the legs and the mid-ventral region of the abdomen. Mortality was usually less on oilseed rape than Chinese cabbage and turnip. These observations suggest that fungistatic compounds of oilseed rape were interfering with the infection process.