At one sampling site an entomopathogenic fungus, tentatively identified as Tolypocladium cylindrosporum, was found to severely reduce populations of hibernating turnip moth larvae (Agrotis segetum) in consecutive winters, while it was never recorded from other collection sites. It produced cylindrical conidia (5·5×2 μm) and smaller, ellipsoidal conidia (3·8×2·8 μm) in slimy heads from flask-shaped phialides with thin necks, which occasionally were bent. Phialides were scarce and never occurred in true verticils. Colonies were pale yellow, cottony on artificial medium, with a characteristic mouldy smell. The infectivity of the fungus was tested in a semi-field trial, where suspensions of conidia were applied to hibernating turnip moth larvae in plastic buckets. The experiment also included buckets with larvae and/or soil from the infested site in order to study the natural fungal-induced mortality. The experiment was started in mid-October and buckets were collected four times from January until May. The fungus had caused high mortality by January, when almost 70% of the treated larvae were infected and by March more than 94% of the larvae had been killed. For larvae from the naturally infested site the prevalence was 21% in mid-October and by March it had reached 93%. The ability of the fungus to infect insects at low temperatures may be related to a relatively low optimum temperature for growth of 21 °C, which is lower than for the majority of entomopathogenic hyphomycetes. Host range studies with 10 insect species indicated that the fungus is probably specific to hibernating cutworms.