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The potential for entomopathogenic nematodes to control flies in cattle feedlots was determined by screening 40 strains representing 8 species of Heterorhabditis Poinar and 5 species of Steinernema Travassos for virulence toward 3rd-instar house flies (maggots), Musca domestica L. None of the 22 strains of Heterorhabditis infecting maggots caused significant levels of mortality in a filter paper assay. Ten strains of Steinernema infected maggots, of which 7 strains--4 S. carpocapsa (Weiser), 2 S. feltiae (Filipjev), and 1 S. scapterisci (Nguyen & Smart) caused significant mortality. Ten Heterorhabditis strains and 10 Steinernema strains successllly reproduced for 22 generations in maggots. No difference was observed between 72-h survival of maggots and adult emergence. Six strains of Steinernema were selected for 10 generations on maggots and then compared with unselected lines. No difference in pathogenicity between selected and unselected lines was observed. Two strains of S. feltiae -- SN and UNK-36 -- and 2 of the best Heterorhabditis -- H. bacteriophora Poinar OSWEGO and H. megidis Poinar, Jackson & Klein HF-85 -- were tested in a fresh bovine manure substrate. All 4 strains produced significant fly mortality in the manure substrate, although the S. feltiae strains had significantly lower LC50 values than did the Heterorhabditis spp. The most promising strain, S. feltiae SN, gave LC50 and LC99 values of 4 and 82 infective juveniles per maggot, respectively. These doses were equivalent to 2.7 and 55 infective juvenile per gram of manure and 5.1 and 104 infective juveniles per square centimeter of surface area. Infective juveniles capable of infecting greater wax moth larvae, Galleria mellonella (L.) survived in manure for up to 10 wk without hosts.