AbstractParasitic nematodes do not biosynthesize sterolsde novo and therefore possess a nutritional requirement for sterol, which must be obtained from their hosts. Consequently, the metabolism of phytosterols by plant‐parasitic nematodes is an important process with potential for selective exploitation. The sterol compositions of several species of plant‐parasitic nematodes were determined by capillary gas chromatography‐mass spectrometry and compared with the sterol compositions of their hosts. Saturation of the phytosterol nucleus was the major metabolic transformation performed by the root‐knot nematodesMeloidogyne arenaria andM. incognita and the corn root lesion nematode,Prytalenchus agilis. In addition to saturation, the corn cyst nematode,Heterodera zeae, dealkylated its host sterols at C‐24. Because free‐living nematodes can be cultured in sterol‐defined artificial medium, they have been successfully used as model organisms for investigation of sterol metabolism in plant‐parasitic nematodes. Major pathways of phytosterol metabolism inCaenorhabditis elegans, Turbatrix aceti andPanagrellus redivivus incleded C‐24 dealkylation and 4α‐methylation (a pathway unique to nematodes).C. elegans andT. aceti introduced double bonds at C‐7, andT. aceti andP. redivivus saturated the sterol nucleus similarly to the plant‐parasitic species examined. Several azasteroids and long‐chain dimethylalkylamines inhibited growth and development ofC. elegans and also the Δ24‐sterol reductase enzyme system involved in the nematode C‐24 dealkylation pathway.