An estuarine snail (Batillaria attramentaria), introduced to northern California marshes, is displacing a native confamilial mudsnail (Cerithidea californica) through superior competition for shared, limiting food resources. Batillaria, however, is absent from similar marsh habitats in southern California. I tested whether regional-scale variation in relative performance (growth) of the snails may have influenced Batillaria's invasion pattern. I quantified growth using RNA:DNA ratios (a growth index that I ground-truthed with direct growth measurements) for snails collected throughout their entire collective North American distribution. Batillaria exhibited a high growth rate that was more than double Cerithidea's growth rate in sympatric populations. A broad-scale relationship of species' growth rates against latitude projected an amply adequate growth rate for Batillaria in southern California where it is presently absent. Furthermore, growth rates of Cerithidea did not increase in southern California, suggesting that Batillaria would maintain its dramatic relative performance advantage. Thus, even if resources are limiting at southern latitudes, biotic resistance through competition with Cerithidea does not explain Batillaria's absence. Among alternative, untested hypotheses for Batillaria's absence, insufficient propagule inoculation has strongest support. Because transplant experiments with nonindigenous species are unethical, examination of species' performance over geographic scales provides a powerful alternative approach for invasion studies.