The life cycle and population dynamics of the entomopathogenic nematodes Steinernema carpocapsae and S. feltiae were studied in monoxenic liquid culture with their symbiotic bacteria Xenorhabdus nematophila and X. bovienii. To distinguish between the different juvenile and adult stages, their size was recorded. No differences were observed between the species in the size of the juvenile stages but significant differences were recorded in the length of the F1 adults, pre-dauer (J2d) and dauer juvenile stages (DJ). On average, 90% of inoculated DJ of S. feltiae recovered and 77% of S. carpocapsae. In general, S. feltiae developed from the inoculum DJ to the adult approximately 1 day faster than S. carpocapsae. The sex ratio was female-biased (59.2 ± 2.2% in S. carpocapsae, 66.7 ± 2.6% in S. feltiae) in the parental population but not in the F1 generations. Steinernematid adults, like heterorhaditids, respond to depleting food resources with the cessation of egg laying. Juveniles hatch inside the uterus and develop at cost of the maternal body content causing the death of the adult (endotokia matricida). In contrast to Heterorhabditis spp. and in vivo observations of steinernematids by other authors, who reported that readily developed DJ leave the carcass of the dead adult, in this study J2d emerged 12 h after cessation of egg laying. The density of both bacterial cultures decreased due to the feeding of the parental juveniles. However, X. nematophila continued at very low density, whereas the density of X. bovienii increased again until 15 days post-inoculation. The vast majority of F1 S. carpocapsae offspring developed to DJ, whereas in S. feltiae a significant second and third generation of adults was observed, probably due to the increasing bacterial population. However, second and third generation adults in S. feltiae cultures did not contribute significantly to the DJ yield. Mean yields of 158 × 103 DJ ml–1 were recorded for S. carpocapsae and 106 × 103 DJ ml–1 for S. feltiae. The results provide valuable information for future process improvement.