The total lipid (TL) content and the fatty acid composition of the different lipid classes in infective juveniles of Steinernema feltiae and Heterorhabditis megidis isolates from Greece (GR) and from the United Kingdom (UK) were compared, and the changes during storage were determined. Steinernema feltiae GR had a greater amount of TL than S. feltiae UK, which was reflected in a greater neutral lipid (NL) content. The phospholipid and free fatty acid fractions of the two Steinernema populations were similar. The two Heterorhabditis populations did not differ in their lipid content. In both S. feltiae populations, NL decreased by ca 60% during the first 4 weeks but remained more or less stable during the next 16 weeks. In the two Heterorhabditis populations, NL decreased by ca 70% during the first 4 weeks and continued to decrease, with 3-5% remaining after 12 weeks. The fatty acid profiles and the percentages of saturation of NL and phospholipid fractions in each population were similar before and after storage.