delta7- and delta5,7-Sterols are converted to the corresponding delta5-sterols by Phytophthora cactorum. When delta7-sterols were supplied to the fungus, delta5- and delta7-sterols were recovered in an acetone extract of the mycelium, the proportion of delta5-sterol being higher in the free sterol fraction than in the esters. When delta5,7-sterols were supplied, delta5-sterols were recovered. No delta5,7-sterols were found in the mycelium. The initial rate of uptake of the sterols was in the order delta5 greater than delta7 greater than delta5,7.