Persistence in soil of ten isolates of three species of Heterorhabditis was monitored by baiting the soil with insects and recording their mortality. Infective juveniles (IJ) of the same ten isolates were also incubated in tap water and their survival recorded. Both tests were conducted in the laboratory at 20°C. Inter- and intraspecific differences in persistence were detected: H. bacteriophora HI was the most persistent isolate in both media. There was no clear division between H. megidis (North West European Group) and H. downesi, but isolates of H. downesi tended to be less persistent than those of H. megidis. Nematodes persisted longer in soil than in water: after 180 days in water, all IJ were dead in seven of the ten isolates, whereas all isolates still killed insects after 265 days in soil. Persistence of isolates in soil (indicated by LT50, the time that nematode-infested soil retained the ability to kill 50% of the bait insects) was correlated with their survival in water (represented by ST50, the time at which 50% of the IJ were still alive), with r2 = 0.84, indicating that similar factors were responsible for the reduction in each parameter.