In the zygomycete Phycmyces blakesleeanus the biosynthesis of p-carotene is under the control of blue light.The light-controlled expression of the cam gene encoding phytoene dehydrogenase was investigated with slot blot and Northern analyses After irradiation of mycelia with short pulses of blue light the amount of cad3 mRNA was stimulated transiently in the subsequent dark period. Depending on the energy fluence of the light pulse the mRNA increased abut 3 to 5-fold after a latency of 2-5min. Twenty minutes after the irradiation the amount of cad3 mRNA decreased again and reached, after 60 min, the dark level it had prior to the irradiation. A Northern analysis showed that the carB mRNA is unstable. After irradiation the fulllength mRNA (2.2 kb) as well as degraded carB mRNA increased. The degradation was specific for the cam gene, because no degradation was observed for 25s rRNA or the mRNA of the p y a gene. The rapid turnover as well as the degradation of the cad mRNA provide for an adaptive light control and appear to represent an essential feature of the regulation of the carotene pathway.