Methyl [(17L)-17-3H1]stearate, methyl [(17D)-17-3H1]stearate, and methyl [(17DL)-17-3H1]stearate have been prepared. Each of these compounds, mixed with methyl [U-14C]stearate, has been incubated with Torulopsis gropengiesseri, to give, after work-up, methyl 17-L-hydroxystearate and dimethyl octadecane-1,18-dioate. Determination of the 3H : 14C ratios of the latter compounds has established (a) that ω-hydroxylation and ω-1-hydroxylation of stearic acid are independent reactions which involve overall a direct substitution of a hydrogen atom by a hydroxy-group, (b) that the ω-1-hydroxylation of stearic acid is a stereospecific process which takes place with retention of configuration, and (c) that a kinetic isotope effect probably operates during the hydroxylation of [(17L)-17-3H1]stearic acid. A mixture of 1-bromo[(16DL)-16-3H1]heptadecane and 1-bromo[U-14C]heptadecane has been oxidised with T. gropengiesseri, to give, after work-up, dimethyl heptadecane-1,17-dioate. No change in the 3H : 14C ratio was observed during this transformation. This result has been interpreted as evidence that alk-1-enes are not intermediates in the terminal oxidation of alkanes by the yeast.