This project used genetic variation to investigate dispersal, inbreeding and social behaviour in the aphid $Pemphigus\; spyrothecac$. $\backslash textit\{$\textit{P. spyrothecae}$\}$ is cyclically parthenogenetic, reproducing sexually on the bark of its primary host, $Populus\; nigra$, and asexually within galls on the leaf petioles. Within the gall, a soldier caste defends and cleans the gall, potentially reducing its own fitness. $P.\; spyrothecae$ collected across the UK from 1997 to 1999 and from mainland Europe and America in 1999 were genotyped using seven variable microsatellite markers that I developed in collaboration with William Amos and Kate Llewellyn. Using population genetic analysis, I showed that $P.\; spyrothecae$ populations were temporally stable over three years, and spatially structured. Populations from trees 5 to 1700 km apart were significantly differentiated, and loosely followed an isolation-by-distance model. There was slight evidence of differentiation between neighbouring trees (5 to 500 m apart), but not between samples taken from within trees (less than 5 m apart). By contrast, $P.\; bursarius$, a closely related species that, unlike $P.\; spyrothecae$, has a secondary host, showed no differentiation between populations 150 km apart, suggesting that population isolation in $P.\; spyrothecae$ may be a consequence of losing its secondary host. Populations within trees were highly inbred, probably due to selfing between sexuales from the same clone. This finding corresponds with the theory that female-biased sex ratios in P. spyrothecae evolved through local mate competition. There was no evidence for a correlation between inbreeding and population density. Genetic variation was also found within galls. Of 633 aphids in one gall, 619 shared one genotype, while the remaining 14 were immigrants from at least nine other clones. One immigrant was found among 49 aphids from four other galls. Such a low level of clonal mixing probably favoured the evolution of soldiers, and may represent an investment in dispersal by the clone as an insurance against its death.