1. To determine whether dispersal biology can predict the pattern of population-genetic variation among insect populations accurately, allozyme variation was assayed for populations of a saltmarsh planthopper, Tumidagena minuta, in which > 99% of the adults are flightless. 2. The pattern of genetic isolation by distance in T. minuta was compared with that in other insects, to determine whether it was similar to isolation by distance in other sedentary insects. 3. In contrast to predictions, the pattern of isolation by distance in T. minuta was most similar to that seen in the most mobile insects in a recent review of population-genetic variation in insects. Furthermore, population-genetic subdivision over a spatial scale of > 400 km was weak. 4. Possible causes of the apparent contradiction between dispersal biology and population-genetic structure in this species are discussed. The results for T. minuta highlight the fact that although mobility is generally correlated with gene flow in insects, studies of population-genetic variation must be combined with direct studies of dispersal to understand fully the degree to which populations exchange individuals.