The effect of kinship on growth and use of space by individually PIT-tagged 1+ brown trout was studied for 11 weeks in eight stream enclosures. Each enclosure consisted of two sections, separated by a region containing PIT-detecting antennae, which enabled us to measure use of sections by all individuals. Two types of sibling groups were tested, a single sibling group, F1, consisting of four individuals that were reared together in hatchery tank 'a' (F1a) plus four additional siblings of the same family but raised in hatchery tank 'b' (F1b), and a mixed sibling group, consisting of four F1a individuals plus four siblings from a second family, F2. Based on kin theory and earlier laboratory studies, we expected that growth of the F1a individuals in the single sibling group to be greater than that of F1a individuals in the mixed family sibling group, but instead we found just the opposite. The variance of growth did not differ between treatments. Nor was there a difference in time F1a individuals spent together when they were in mixed versus single sibling groups. We did find that F1a individuals changed habitat more frequently than F2 individuals in the mixed sibling group but less frequently than F1b in the single sibling groups. Thus, our predictions based on kin theory for growth and behavior of brown trout were not supported by our data, and we suggest that the role of kin recognition for the ecology of salmonids deserves further attention.