The aerodynamic diameter, da, of conidia produced in vivo and in vitro by the entomopathogenic fungus Erynia neoaphidis were estimated using an impaction method. The estimated values of da for conidia produced in vivo were smaller than those produced in vitro: in vivo the values of da for primary and secondary conidia were between 16 and 18 μm (equivalent fall speed, Vs, 0·8–1·0 cm s−1); for in vitro produced conidia da values were between 28 and 31 μm (Vs, 2·4–3 cm s−1). For conidia produced by field collected cadavers the value of da, was estimated to be similar to that for conidia produced in vivo in the laboratory. The aerodynamic diameter of primary conidia of Conidiobolus obscurus (strain X39) and Zoophthora radicans (strain NW250) produced in vivo were also measured using the same method. The values of da for these two species were 45 and 17 μm (Vs, 6·2 and 0·9 cm s−1) respectively. Implications for dispersal of E. neoaphidis are discussed. The physical diameters of the test spores were measured microscopically and compared with the aerodynamic diameters.