Although kin associations between fish can develop in the laboratory, the importance of kin structure in natural social groups remains uncertain. Guppies (Poecilia reticulata) possess many behavioural and life-history traits that promote kin structure, and so constitute ideal candidates for testing its general significance amongst fish. We collected 25 adult only shoals from similar to150 m stretches along two Trinidadian rivers. The 272 sampled fish were genotyped from a pool of seven hypervariable microsatellite loci. Although mark-recapture experiments have previously detected male-biased dispersal, which is predicted from the guppy mating system and which may have important implications for population differentiation, we found no sex-biases in dispersal between shoals. Because sex-biased dispersal was not evident, males and females were pooled in analyses of kin structure. Relatedness levels within each shoal did not differ significantly from zero, indicating that kin selection does not contribute to gross structure. Instead, our data is more consistent with the hypothesis that individual-level selection underlies shoal composition dynamics. These dynamics have important implications for the evolution of cooperative behaviour in guppies.