Dioscorea deltoidea cell suspension cultures were established in modified Murashige and Skoog medium. The diosgenin production increased from 0.10 mg g(-1) to 3.98 mg g(-1) dry cell weight when cells were cultivated in the light and in a growth medium limited in phosphate and sucrose. The addition of 1.3 g of autoclaved fungal mycelium of Alternaria tenuis per litre of cell culture growing in the dark induced the production of 0.04 mg diosgenin g(-1) dry cell weight. In both cases, the production of diosgenin was preceded by a transient induction of isopentenyl diphosphate isomerase activity.