Social behaviour is critical for survival and fitness in many organisms. The aim of this study was to examine the effects of developmental state, physiological state and their differences between individuals on the intensities of aggressive and grouping behaviours in red drum (Sciaenops ocellatus) larvae. Body size was used as a proxy for developmental state and body condition (i.e., residual of wet weight from the relationship between body size and wet weight) as a measure of physiological condition. Social interactions between two fish were observed for 30 min in a 9-l tank. We found that the duration of grouping behaviour increased as body size increased, but aggressive behaviour was less frequent as body size and condition increased. The larger fish in a trial tended to be aggressive to the smaller one, and fish of similar sizes tended to show grouping behaviour. Fish that more frequently displayed aggressive behaviours tended to occupy the lower part of the water column. Body size and condition were important determinants of social interactions between red drum larvae. Our results suggest competitive disadvantage for the larvae from late cohorts when they recruit to the nursery habitats where larger larvae from early cohorts have already occupied.