A host of abiotic factors modify fish social behavior. However, few studies have characterized the effects of temperature on behavior. In this study, brown trout Salmo trutta fry were reared at 5 different temperatures (4 C, 6 C, 8 C, 10 C, and 12 C). In order to characterize group structure, 3 be-havioral parameters were investigated: group social structure (based on inter-individual distances), inter-individual relationships (based on physical contacts), and individual activity. These behavioral parameters were studied at the emergence stage, which corresponds to a switch from a social gregarious life in the gravel to a solitary one in the water column. Data analysis showed that the inter-individual distances increased with increasing temperature, particularly the nearest neighbor distance. The mean number of physical contacts between fry increased at both low and high temperatures. At high temperatures, most fry moved apart from each other after a physical contact. Swimming activity decreased at both the lower and upper temperatures (18% of activity at 4 C, 38% at 8 C, and 12% at 12 C). This study showed that temperature modifies brown trout fry activity, inter-individual relationships, and social behavior, which all affect group cohesion before emergence and can influence their survival and dispersal.