Four isolates of the entomopathogenic fungus Erynia neoaphidis, from different hosts and locations in Switzerland, were distinguished by their DNA fragment banding pattern obtained by RAPD–PCR. The isolates differed in sporulation and germination rates, but not in virulence (overall mean LD50 16 conidia mm−2) against the aphid Acyrthosiphon pisum. The sporulation rate was 6 to 24 conidia mm−2 10 min−1. On nutrient agar, 24–73% of the conidia produced secondary conidia and 32–68% produced germ-tubes on polystyrene. Differentiation of primary into secondary conidia or germ-tubes is possibly dependent on the presence of free water on the contact surface; on a dry surface, conidia germinated mostly with a germ-tube, whereas on a wet surface the conidia produced a secondary conidium. Experimental support is provided for the claim that, in addition to LD50 values, features such as sporulation rate, spore germination, capacity to form secondary conidia and intraspecific variation should be considered in selecting isolates of E. neoaphidis for biocontrol agents.