A photoinduced endogenous rhythm of zonation has been described by Jerebzoff and Piskorz-Binczycka (1987) and Piskorz-Binczycka et al. (1989) in the fungus Pénicillium claviforme Bainier CBS strain 126-23 ; a 24-26 h periodicity was elicited by and displayed in continuous white light. The present work shows that aspartate and alanine aminotransferase activities oscillated in continuous white light (fluence rate: 1.5 mW m -2 ), in synchronism, with a period of about 24h.