I examined spatial synchrony in populations of monarch butterflies (Danaus plexippus) during the summer breeding season across North America and while overwintering along the Pacific Coast. Spatial synchrony was observed in all analyses, but was particularly great among eastern summer populations and among overwintering populations on the Pacific Coast. Thus, in a year when relatively large numbers of monarchs were found at a particular breeding or wintering site in these populations, other sites within a wide area were likely to have relatively large numbers of monarchs as well. Spatial structure, measured by the decline in synchrony with distance, was relatively strong among summer populations in eastern North America, weaker, but present, among western summering populations and virtually nonexistent among overwintering populations along the Pacific Coast. Spatial synchrony in mean temperatures during the relevant time periods was generally much greater than that observed in the monarchs, consistent with the hypothesis that environmental factors (the Moran effect) may be playing a central role in causing the observed population synchrony. However, differences in spatial structure between monarch populations and mean temperature during the summer suggest that other factors, possibly post-migratory dispersal, are also affecting synchrony. Dispersal is particularly likely to be important in determining the unusual lack of spatial structure observed in overwintering populations of monarchs, which are believed to mix over wide areas during fall migration.