Bacterial (Enterobacteriaceae) symbionts isolated from newly collected Steinernema glaseri (insect pathogenic nematodes) were found to differ from those previously isolated in being able to produce antibacterial compounds. They were identified as Xenorhabdus nematophilus subsp. poinarii. Study of the new strains showed that the dimorphism common to Xenorhabdus spp. is expressed differently in X. nematophilus subsp. poinarii and that this subspecies differs from other X. nematophilus in several other respects. X. nematophilus subsp. poinarii produced colony variants that did not differ from the parent strain in antibacterial activity whereas some strains lost antimicrobial activity without changing colony morphology; no variant of any strain absorbed bromothymol blue from agar media. There was also considerable variation in colony morphology between different isolates. Moreover, although the nematode/bacterium complex was an efficient pathogen, X. nematophilus subsp. poinarii strains were unlike other X. nematophilus in not being highly pathogenic when injected intrahaemocoelically into insect larvae (Galleria).