Cultures of Saccharomyces cerevisiae strain GL7 (a sterol auxotroph) were incubated with nonradioactive and tritium-labeled cycloartenol, and the sterol composition of the cells was examined by chemical (GLC, TLC, HPLC, MS, 1H-NMR, and 13C-NMR) and radiotracer techniques. Several novel sterols were isolated from the cells including 14α-methyl ergosta9(11),24(28)-dien- 3β-ol, 24β-methyl-9β,19-cyclopropyl ergost-8(14)-en-3β-ol, and 9β,19- cyclopropyl ergosta-7(8),24(28)-dien-3β-ol. GL7 converted [2- 3H]cycloartenol to [2-3H]ergosterol in low yield (1% incorporation), whereas [2-3H]lanosterol was converted to [23H]ergosterol in high yield (41% incorporation). The degree of sterol absorption and transformation by GL7 was dependent on the type and amount of sterol(s) in the growth medium. The results demonstrate for the first time that yeast may transform 9β,19- cyclopropyl sterols to 9(11)-sterols and Δ5-sterols and that 14- demethylation of sterols may proceed in GL7 to double bond formation either in the 8(14)-position or in the 14(15)-position.