We describe the genetic structure of a freshwater insect species and interpret it in terms of present-day population dynamics and possible postglacial colonization history. The sampling regime represented a large area of the species range in northwest Europe, particularly focusing on Britain, a region relatively neglected in molecular population genetic studies. Plectrocnemia conspersa generally showed low levels of genetic variation across the sampled populations (Nei’s D = 0.0138) and subdivision was unrelated to the pattern of the drainage network. However, the results do suggest that populations across the region are not at equilibrium and that British populations still show effects of the recolonization of the species following the last glacial maximum. Levels of genetic diversity were lower in Britain than in mainland Europe. Two-dimensional scaling showed genetic differentiation between major regions and the pattern of genetic diversity indicates a more recent origin of populations in the north and west of the area compared with the south and east. We argue that, despite the highly fragmented larval habitat, dispersal over tens of kilometres is frequent. Over longer distances, however, P. conspersa does still show evidence of founder effects and postglacial range expansion into Britain.