The relationship among relative size, growth rate and diel visits into a feeding compartment were studied in Arctic charr Salvelinus alpinus, a species known for its flexible activity patterns. Individual swimming activity from a refuge compartment into an uncovered feeding compartment was studied using an automated passive integrated transponder system. Approximately half of the individuals in four groups of S. alpinus spent 70–80% of their time in the feeding compartment, regardless of the time of day. The remaining individuals spent nearly as much time (70–80%) in the feeding compartment during the night, but only spent c. 20% of their time in the feeding compartment during the day. These individuals had a lower mean ±s.d. masses (13·17 ± 4·34 g) and growth rate (daily growth coefficient, G= 0·80 ± 1·19) than those individuals that spent most of their time in the feeding compartment during the day (mass = 16·65 ± 5·73 g and G= 2·04 ± 0·81). This indicated that some smaller fish were absent from the feeding compartment during daytime, possibly to avoid aggression from larger conspecifics. In the open compartment, the aggressive interactions were more frequent in daylight than during night and always low in the covered compartment.