Third-instar larvae ofMusca domestica, Ophyra aenescens, Fannia canicularis, andFannia femoraliswere exposed to parasitic nematodes in the laboratory. Four strains ofSteinernema feltiae, two ofSteinernema bibionis, and one ofHeterorhabditis heliothidiswere tested on moist filter paper in petri dishes. Considerable variability in infectivity within and among hosts was noted for the different nematode strains. Strains ofS. feltiaewere more infective thanS. bibionisorH. heliothidis, but all could infect the four hosts.F. femoraliswas highly resistant to infection byS. bibionisandH. heliothidis.Further tests on chicken manure and fly rearing medium showed that infectivity of several nematodes forM. domesticaandF. canicularislarvae was markedly less than on filter paper.