Bioassays were initially conducted in Petri-dishes to screen the efficacy of four Heterorhabiditis and Steinernema species against the mushroom phorid Megaselia halterata. Control rates of 61 to 70% control were obtained at a dosage of 1500 infective juveniles (IJs) per 30 larvae. In order to avoid stress-induced susceptibility, an improved bioassay system in micro-wells, filled with 0.6 ml of compost agar and 0.2 of compost colonized by Agaricus bisporus, was developed. In a screening of different species of Heterorhabiditis and Steinernema with applications of 30 IJs per phorid larva, a highest parasitization rate of 20% was obtained with S. feltiae. Bioassays were continued with S. feltiae in dosage-mortality assays in which larvae of the sciarid Lycoriella auripila and the phorid M. halterata were challenged. At the lowest dosage of 30 IJs per sciarid larva, 78% control was obtained. Increasing the dosage from 30 to 1000 led to only small increases of the phorid mortality. At 1000 IJs per larva a significant mortality of 18% was obtained. The nature of the substrate, compost or casing did not greatly influence the parasitization rates. The sex ratio of nematodes that were able to penetrate and establish in the phorid larvae appeared to be female-skewed. Males were only present at a mean of 19%. Low susceptibility of the phorid larva was ascribed to the inaccessibility of its small mouth opening.