The role of light intensity in the entrainment (synchronisation) and the shortest entrainable period of the circadian conidiation rhythm ofNeurospora crassawere tested by growing the fungus on solid media under different light‐dark‐regimes. The mutant strainsbd, csp‐1‐bdandfrq‐1‐bdcould be entrained (synchronized) to a period of 12 h. The light phase of this period was inversely correlated with the log of the light intensity. Enhancing the irradiation to 3.65 W/m2white light, the mutantcsp‐l‐bdcould be entrained by as short a period as 2: 2 h. An alteration in the effective wave lengths could not be observed. These results question a total identity between this rhythm and the biological clock found in other eucaryotic organisms.