Estimates of the economic impacts of nonnative nuisance (“invasive”) species must rely on both a sound ecological understanding and the proper application of economic methods. Focusing on the example of the invasive European green crab (Carcinus maenas), we show that the crab's estimated economic impact—which has been used to help justify recent public policy—is based on data taken from the wrong geographic location. Furthermore, the predictions of ecological effects appear to rest on loose footing, and economic methods have been misapplied in constructing the estimate. Our purpose is to call attention to the need for the more careful application of science and economics in managing this pressing environmental issue.