This study is aimed at evaluating the likelihood that kin-biased distribution will be expressed or detected in a range of brown trout (Salmo trutta) populations as a function of family size. Microsatellite analysis indicated that fewer full- and half-siblings were found in populations with larger effective population sizes, while more full- and half-siblings were found in populations with lower effective population size. It is suggested that kin-biased distribution and hence kin-biased behaviours are not likely to be expressed equally frequently in all populations since the number of close relatives will vary among populations and, hence, the opportunity for relatives to interact differs among populations. These findings can, at least partly, explain the discrepancy among previous studies of kin-biased distribution in wild salmonids under natural conditions. Effective population size could, hence, be used to predict the salmonid populations in which kin-biased distribution are more likely to occur and be detected.