Invasive species cause widespread ecological and economic damage, but to better understand how exotic species spread to become invasive, empirical studies of the mechanisms driving phenotypic differentiation between populations of invasive species are required. This study determined whether differences in distal spine length among populations of Bythotrephes longimanus in lakes with or without gape-limited fish predators could be explained by local adaptation or phenotypic plasticity. I collected Bythotrephes from six lakes and found that distal spine lengths and natural selection on distal spine length differed among populations, but were unrelated to the gape-limitation of the dominant fish predator. A common garden experiment revealed significant genetic and maternal variation for the trait, but phenotypic differences among populations were not genetically based. Phenotypic differentiation of this ecologically important trait is, therefore, a result of plasticity and not local adaptation, despite spatially variable selection on this heritable trait.