The influence of photoperiod, of feeding frequency and of a water temperature cycle on the formation of daily microgrowth increments was studied in lapilli of juvenile Tilapia mariae. These exogenous factors did not affect the production of daily increments, suggesting the existence of a circadian endogenous rhythm. The characteristics of the increments were affected by the experimental conditions, suggesting that the endogenous rhythm promotes a circadian cycle of activity or rest, which results in the formation of daily increments.