The effect of simulated rain on the persistence of oil and water formulations of conidia of the entomogenous fungus Metarhizium anisopliae when applied to oilseed rape foliage was investigated, using third instar larvae of the mustard beetle (Phaedon cochleariae) as the target host. Rain significantly (P < 0.01) reduced the susceptibility of the beetle larvae to M. anisopliae but the amount of inoculum removed was influenced by the formulation. Larvae exposed to plants treated with conidia formulated in aqueous Tween, Shellsol T, or sunflower oil/Shellsol T resulted in 55, 82.5 and 72.5%, mortality, respectively. The mortality for these respective formulations was reduced by 42, 57 and 51% if the plants were exposed for 1 h to simulated rain. Laboratory and field studies showed that more inoculum collected beneath plants sprayed with conidia formulated in Shellsol T or aqueous Tween than in the more viscous sunflower/Shellsol T mixture. Mortality studies on leaves taken from field plots suggested that conidia on leaf surfaces could be replenished by repeated application. The number of conidia isolated from field plots was greater where inoculum was applied bi-weekly than once weekly.