Aerobic mycelium of wild-type Mucor rouxii accumulated about ten times higher amounts of the carotenoid pigment beta-carotene when grown continuously in the presence of light than the corresponding cultures grown in the dark. Carotenoid accumulation was dependent on light intensity, with the threshold located at about 10(-2) W.m(-2). Photocarotenogenesis in complex medium was more efficient with glucose as a carbon source. Carotenoid synthesis by M. rouxii mycelium was unaffected by both retinol acetate and retinal, which are stimulators of carotenogenesis in other zygomycetes. Carotenogenesis was significant in aerobic mycelium but was almost undetectable in anaerobic mycelium as well as in aerobic or anaerobic yeast cells. This suggested an involvement of oxygen in carotenoid synthesis by M. rouxii and the existence of developmental regulation of the expression or operation of the pathway.