The temporal pattern of release and dispersal of inoculum of plant and insect pathogenic fungi play an important role in the spread of disease. Airborne concentrations of primary and secondary conidia of Erynia neoaphidis released from the rose-grain aphid Metopolophium dirhodum were monitored at the edge of two winter wheat crops on IACR-Rothamsted Experimental Farm between May and September in 1996 and 1997. Hourly average temperature and humidity were recorded at each spore trap site and daily totals of rain and sunshine hours and daily average wind speed recorded about 1.6 km from the monitoring sites. No airborne conidia were found in 1996, but large numbers were trapped at the two sites in 1997. They were present from mid-June until early August, reaching peak concentrations on 17–18 July. Concentrations were usually highest during the night and in the early morning (01:00–07:00 h GMT) and were generally low during the day. On the 3 days when significant numbers of conidia were caught in the afternoon, daytime relative humidity was high (about 89%) and day-time temperature low (about 16 °C). Night-time conditions nearly always favoured the production of conidia. This suggests day to day variation in airborne conidium concentrations may be affected more by underlying biological factors than environmental conditions.