[Methyl-¹⁴C] methionine was supplied to yeast cells under aerobic and anaerobic conditions for the investigation of the pathway for ergosterol biosynthesis after the methylation of the side-chain. Under aerobic conditions, the incorporation of radioactivity into ergosterol was high. With a limited oxygen supply, in contrast, the radioactivity was first accumulated in ergosta-7, 24 (28)-dien-3β-ol and ergosta-8, 24 (28)-dien-3β-ol, and then transferred to ergost-7-en-3β-ol, ergost-8-en-3β-ol and ergosta-7, 22-dien-3β-ol with time. Under strictly anaerobic conditions, a double bond was introduced neither to Δ⁵ nor to Δ²². The results of the tracer experiments suggested the operation of several pathways in the late stages of ergosterol biosynthesis. It was also suggested that the main pathways varied depending on the conditions such as oxygen supply and other factors. The above conclusion was supported by the results of the analyses of the sterol compositions of the cells grown under various conditions.