The effect of different photoperiod regimes, as well as cover in continuous light (24L/0D) and darkness (OL/24D), on the growth rate of larval Clarias gariepinus was evaluated. In addition, a behavioural study of the effect of light on larval and juvenile C. gariepinus was made. The growth rate of larvae was found to increase with shorter light periods, the highest being recorded in continuous darkness (0L/24D photoperiod). Cover was found to enhance the growth rate of larvae reared under a continuous light period, however, the presence of cover did not affect the growth rates of larvae reared in continuous darkness. The behaviour of larval and juvenile catfish differed markedly in continuous light and darkness. When subject to a continuous light period, a strong negative phototaxis, long rest periods interspersed with disturbed activity, refuge seeking behaviour, and increasing incidence of territorial aggression were observed. In continuous darkness, the fish displayed a high level of active swimming with fewer rest periods, and a lower incidence of disturbed rest activity. The level of territorial aggression among fish in continuous darkness gradually declined and became negligible. Higher levels of browsing and airbreathing activity were observed in continuous darkness than in light. The present results, interpreted together with other ecological and anatomical data, demonstrate this species to be primarily a nocturnally active, tactile feeder, with a distinct diet activity pattern. The refuge seeking behaviour observed in the light is interpreted as an adaptation to avoid predators that rely on sight to detect prey. This behaviour, however, appears to promote intraspecific competition for refuges, resulting in a high degree of territorial aggression, and probably an increase in the rate of cannibalism. With regard to fish culture, it is suggested that the rearing of larvae in conditions of continuous darkness or low light intensity is optimal owing to the higher growth rates and apparent reduction of stress observed under these conditions.