The dynamics of a recent outbreak of the spruce bark beetle (Ips typographus) in Switzerland was ruled by a devastating winter stormin 1999 and the drought and heat of the summer 2003. Starting from a similar level of population sizes, estimated as the rate of infested growing stock, beetle populations increased differently inmagnitude and time among different regions in Switzerland. Accordingly, we expected local or regional genetic differentiation as a result of such repeated population expansion/breakdown dynamics. We analyzed 5 nuclear microsatellites of spruce bark beetles sampled from pheromone traps at 30 locations distributed over Switzerland. Our genetic results did not indicate any sign of population differentiation, structure, isolation by distance, or recent bottlenecks. This complete lack of genetic structure suggests that spruce bark beetles are highly mobile, precluding the formation of a spatial structure at neutral molecular markers. Thus, this molecular–genetic approach does not allow us to discriminate among regional gene pools and to identify the origin of expanding beetle populations.