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Background. Cannibalism occurs at various ages or sizes depending on fish species and the respective environments. Barramundi, Lates calcarifer (Bloch, 1790), is a popular and valuable species in aquaculture on the Asian and Australian continents. In its culture, cannibalism can cause severe losses during the early stages of development particularly before fish reach a length of about 10 cm. Hence the present study aimed to study the effect of the two photoperiods, constant darkness and continuous light on the rate of sibling cannibalism in barramundi juveniles reared indoors under intensive conditions. Materials and methods. The indoor experimental setup consisted of two separate systems labelled system 1 and 2. Each system consisted of three rectangular rearing tanks each of 60 L capacity connected to a mechanical and a biological water-cleaning unit. System 1 was kept under 24 h of light provided by two 36 watt fluorescent tubes. System 2 was kept under 24 h of darkness except for a short period of 6 min during each feeding time. Each rearing tank was randomly stocked with 187 juveniles. A commercial fish feed was provided to the fish by hand ad libitum four times during the day time. The experiment was conducted for 112 days. Results. In both light and dark systems, larger specimens were much more active during feeding, chasing smaller siblings away from the provided feed, leaving them only the remains to feed on. The accumulated observed mortality was similar under both dark and light conditions. In the present study although feed was provided ad libitum, cannibalism still occurred under both light and dark conditions. However, the rate of cannibalism was significantly (P < 0.05) lower under dark conditions. The results show that a potential predator can swallow a prey up to a maximum of 67% of its own body length. Similar growth performances were observed in the fish groups reared under dark and light conditions. Conclusion. The application of constant darkness is therefore a useful strategy in significantly reducing cannibalism when rearing barramundi. Furthermore, such a strategy also leads to the formation of a more uniform population of fish which is a desire of the grower. [ABSTRACT FROM AUTHOR]