A number of fungal detoxification reactions of sorbic acid have been reviewed. These include decarboxylation to give trans-1,3-pentadiene, esterification to give ethyl sorbate, reduction to give 4-hexenol and 4-hexenoic acid. It was shown that seven Penicillium species could convert sorbic acid into 1,3-pentadiene whilst P. bilaii, P. fellutanum and P. glabrum did not. However, most Eurotium species were unable to bring about this conversion. Considerable differences in the resistance of two isolates of P. crustosum to sorbic acid were found. An isolate from coconut was more resistant than one isolated from hazelnuts. Both sorbic acid and caproic acid (hexanoic) brought about disorganization of the mitochondrial membranes in P. crustosum. It is suggested that these lipophilic acids inhibit growth by interfering with the electrochemical membrane potential across the mitochondrial membranes.