In this paper are presented the results of experiments undertaken to determine what conditions are necessary for the hydroxylation of progesterone during continuous culture ofAspergillus ochraceus NRRL 405. It was found possible to operate with either one or two stages, but because of the inhibitory effect of progesterone on the growth of the microorganism, optimum yields of 11α-hydroxyprogesterone could be obtained only with a two-stage system in which the organism was grown continuously in the first stage and progesterone was added continuously and hydroxylated in the second stage. In addition it was observed that there was an apparent cycling of the microorganism concentration in the period preceding the steady state. No explanation for this was found, and this problem in dynamics is being investigated further. Details are presented of an isotope dilution method using 4-C14-progesterone for determining the concentration of progesterone and its derivatives in fermentation broths.