Conidia ofEntomophthora muscae(Cohn), collected from the onion fly,Delia antiqua(Meigen), produced secondary eonidia at vapor pressure deficits from 0.0 to 3.6, but at highly reduced rates when compared with germination in free water. Germ tubes (the invasive stage) were only produced in the free-water and saturated-atmosphere treatments. Germination rates for these treatments were temperature-dependent; the optimal germination temperature was close to 21°C. The upper and lower conidial germination limits were found to be between 26.7 and 32.2°C and 1.0 and 4.4°C, respectively. Germination and survival in the field were affected by the habitat where the conidia were deposited. Onions, carrots, and grassy border areas (the main sites ofE. muscaesporulation in the onion agroecosystem) affected germination and survival differentially based on the amount of cover in each canopy and the location of the conidia within the canopy. On four experimental dates, conidia experienced nearly 100% mortality after a single day of exposure to full sun. Fungicide applications were detrimental to conidial survival in the field.