The production of gamma-linolenic acid (GLA) by Mucor circinelloides CBS 203.28 and M. rouxii CBS 416.77 in fed-batch cultures operated in pH-stat mode with acetic acid as carbon substrate and titrant compared favourably with the performance of M. circinelloides in batch culture on glucose. On acetic acid M. circinelloides accumulated up to 39.5 mg GLA/S biomass, with a crude oil content of 25% containing 91% neutral lipids. The GLA content of the neutral lipid fraction was 15.6%.