Steinernema affine belongs to the entopathogenic nematodes, i.e. they are able to infect and kill insects (see also: Steinernema). It was orginally described as Neoaplectana affinis from Denmark in 1937 and transferred in 1982 by Wouts et al. to the genus Steinernema Travassos, 1927. The pictures show the infective juveniles (i.e. third-stage juveniles), at this stage the mouth and anus are closed and the pharynx is reduced and non-functional. Often the third-stage juveniles are still enclosed by the retained sheath (part of the old culticle) of the second-stage juveniles. The symbiotic bacteria (Xenorhabdus) are enclosed by an intestinal vesicle, just posterior to the basal bulb. So far this species has been found in Europe and North America.