Gene flow and movement among Hawaiian populations of the diamondback moth,Plutella xylostella (L.), were examined using indirect and direct methods. Estimates of genetic differentiation of subpopulations (FST)based on electrophoretic variation at four polymorphic loci varied from 0.038 to 0.028. Using Wright's method, estimates of the number of migrants exchanged per generation per population (Nem) fromFSTestimates based on four loci ranged from 6 to 9. There was little variation between islands, and only slightly more variation between Hawaiian populations and two populations from the continental United States (Fregion-total= 0.011). The results are consistent with the recent arrival of the diamondback moth in Hawaii. There has probably been sufficient time since its introduction for local populations to have differentiated, but it is less likely that enough time has passed for larger regional population units to differentiate from each other. Results from field experiments in an area surrounding a cabbage plot also suggest that diamondback moth is highly mobile. Average moth density in the 900 m2surrounding the plot was 7.8% of the density of moths within the plot, suggesting substantial emigration out of the plot. The data indicate that local variation in insecticide resistance among Hawaiian diamondback moth populations is not an indication of restricted gene flow and is most probably a result of local variation in selection.