This study was based on naturally occurring and inoculated populations of steinernematid nematodes. The nematode populations were monitored in spring and autumn in 2 consecutive years in an organic cropping system and changes in population size were related to the presence of potential insect hosts. Nematode populations were estimated in terms of nematode incidence (percentage of positive soil samples) by using Tenebrio molitor larvae as bait. The population of naturally occurring nematodes (Steinernema affine and S. feltiae) was generally low (0–17% incidence for S. affine and 0–18% incidence for S. feltiae). Inoculated S. feltiae established well in half of the plots where inoculation had been performed and reached incidences of 87%. Establishment of inoculated nematodes, and population dynamics in general, was clearly influenced by the presence of insect hosts. In crops with high densities of potential hosts (Sitona lineatus in pea and partly Delia radicum in cabbage), nematode incidence increased from spring to autumn, whereas nematode incidence remained unchanged or decreased when few hosts were present (in barley, carrots, alfalfa and leek).