Megaselia halterata (Wood) (Diptera: Phoridae), parasitized by an endoparasitic nematode, Howardula husseyi Richardson, Hesling, Riding (Tylenchida: Allantonematidae), were reared at temperatures of 13, 16, 18, 21, 24, and 27°C. Rearing temperature of 27°C produced a nematode-free phorid culture. Mushroom crops were infested with nematode-free female phorid populations of 1,900, 3,110, 9,800, 12,443, and 12,766 per m2 of growing surface during spawn run. Populations greater than 12,000 phorid females significantly reduced mushroom yield. Two generations of phorids were observed during the cropping period.