Abstract Sterols in the plant-parasitic nematodes Diylenchus triformis and D. dipsaci were analyzed qualitatively and quantitatively. Qualitative analyses were made utilizing gas-liquid chromatography (GLC), thin-layer chromatography (TLC), melting-point determinations, infrared spectra, and quantitative analyses were made by GLC. Major sterols in both D. triformis and D. dipsaci were cholesterol (Δ 5-cholestene-3β-ol) and lathosterol (Δ 7-cholestene-3β-ol). Lathosterol was the major sterol in D. triformis (60%) whereas D. dipsaci contained approximately equimolar concentrations of cholesterol and lathosterol. D. dipsaci sometimes contained C 29 phytosterols, almost entirely in ester form, which were also found in the host (alfalfa callus tissue). These phytosterols were tentatively identified as α-spinasterol and Δ 7-stigmastene-3β-ol, the major sterol being α-spinasterol. The major sterol of Pyrenochaeta terrestris, the fungal host of D. triformis in culture, was ergosterol. Total sterol in D. triformis averaged 0.09% and in D. dipsaci 0.06% on a dry weight basis. Sterol esters averaged 48% of the total sterol in D. triformis and 58% in D. dipsaci.