The present paper was aimed at establishing the time course for development of the light entrainment mechanism of the ERG. Chronic ERG recordings were obtained from juvenile crayfish during different stages of their ontogenetic development, by measuring (as a phase‐response curve, PRC), the effect produced by single 15‐min. light pulses on the period and phase of the ERG amplitude rhythm. It was found that the circadian rhythm of the ERG appears at relatively early stages of development (4th week after hatching) and that the entrainment mechanism developes after the crayfish has reached 2–5 months of age. The role played by the visual photoreceptors in the expression of the ERG rhythm as well as in its entrainment is discussed.