The fatty acid composition of three North West European isolates of Heterorhabditis sp. from different geographical origins, UK211 (England), HF85 (The Netherlands) and EU17 (Estonia) was assessed directly after harvest and, for UK211 and HF85, after 5 weeks storage in water at 20°C. Lipid represented 34–43% of the dry weight of fresh nematodes. Of this, neutral lipid (NL) comprised from 70% (HF85) to over 90% (UK211, EU17). The fatty acid patterns were similar between the three isolates. Oleic (C18:1n-9), palmitic (C16:0), and linoleic (C18:2n-6) acid predominated with 51, 13 and 12%, respectively in the total lipid (TL) of fresh nematodes (average for the three isolates). Levels of unsaturation (U.I.) of fresh nematodes were on average 110, 112, 113 and 152 for the TL, NL, phospholipid and free fatty acid fractions, respectively. EU17 had a slightly lower U.I than the other two strains, despite its more northern origin. Changes in fatty acid composition due to storage were most significant in the NL fraction. The U.I. for the NL fraction increased during storage, suggesting a preferential use of saturated fatty acids.