Induction and characterization of a morphological mutant are described for Heterorhabditis bacteriophora strain HP88. A homozygous inbred line was used as the base population for mutagenesis and genetic analysis of mutations. Mutagenesis was induced by exposing young hermaphrodites to 0.05 M ethyl methanesulfonate. A dumpy mutant (designated Hdpy-l) was isolated from the F generation of the mutagenized population. Morphological studies with light and scanning electron microscopy revealed that the head region of the adult stage was compressed. The head region of the infective juvenile was distorted and the mouth open. Backcross with the original population was successful only between mutant hermaphrodites and wild type males; 50-100 percent of the progeny of this cross maintained the dumpy phenotype, indicating that the ratio between self- and external fertilization of the eggs is > 1 and that the dumpy mutation is recessive.