Lipids are important entomopathogenic nematode nutritional components because they are energy reserves and serve as indicators of nematode quality. The composition and concentration of the media lipid component determine bacterial and nematode yields. Heterorhabditis bacteriophora and its symbiont, Photorhabdus luminescens, were cultured in media containing various lipid sources. As lipid concentration increased from 2.5% to 8.0% (w/v), the final yield and productivity [calculated from the number of infective juveniles (IJ)] increased significantly from 2.1 x 10(5) IJ ml(-1) to 2.8 x 10(5) IJ ml(-1) (P < 0.05) and from 8.9 x 10(5) IJ l(-1) day(-1) to 11.8 x 10(5) IJ l(-1) day(-1) (P < 0.05), respectively. The nematode yield coefficient (IJ per gram of media), however, decreased from 2.8 x 10(6) to 2.2 x 10(6) (P< 0.05), while recovery increased from 45.3% to 58.0% (P < 0.05). Bacterial cell mass remained constant at 4.6 mg ml(-1) with changing lipid content (P > 0.05). The largest nematode yield (2.8 x 105 IJ ml(-1)) was achieved within 8 days, using a medium containing an 8% (w/v) olive and canola oil (50:50 w/v) combination. Moreover, developmental synchrony was achieved in this medium with 96% infective juveniles. In short, lipid sources rich in mono-unsaturated fatty acids and poor in saturated fatty acids produced optimal nematode growth.