Abstract: The effect of different growing substrates on the development of the facultative slugparasite Phasmarhabditis hermaphrodita has been studied in a series of laboratory experiments.Wild, laboratory and Nemaslug strains of P. hermaphrodita were reared in agar plates onhomogenized pig kidney, the homogenized bodies of Deroceras reticulatum, Arion lusitanicus,and Galleria mellonella, the faeces of D. reticulatum and A. lusitanicus, or leaf compost.Development time, yield, lipid reserves, and the body length of females and dauer larvae wereassessed. All P. hermaphrodita strains were able to grow and reproduce on all tested substrates.However, yields were markedly higher on animal substrates. Lipid content and body size variedacross the substrates, however, even plant tissue produced normal sized individuals with normallipid content. It thus seems that the quality of the substrate is expressed mainly in yield. High andless variable yields and faster development of the wild and Nemaslug strains, in comparison withthe laboratory strain, were probably due to different bacterial associates. The dramatic differencesin yields on animal substrates, in comparison to those on plant tissue, illustrate the evolutionaryadvantage of the association of nematodes with invertebrates.