Analysis of genetic variation among populations of stream invertebrates provides a measure of the consequences of effective dispersal, and can be used to determine the extent of movement within and between streams and to infer the likely mechanisms involved. In our recent studies of rainforest stream invertebrates, we have found considerable genetic differentiation among populations of fully aquatic taxa, indicating limited in-stream movement on a very small scale. Adult flight appears to be the principal mechanism of dispersal for aquatic insects; however, analysis of the genetic structure of larval populations of some species also suggests that in-stream movement is limited to a small spatial scale. Furthermore, detailed analysis of the genetic structure of larval populations suggests that recruitment at the reach scale is the result of only a few adult matings and most likely from oviposition by only a few females. We propose that dispersal capability and the stochastic effects of recruitment are key determinants of observed spatial and temporal variation in community structure in some streams.