Abstract Many marine invertebrate species have larvae that depend upon the same organ for both locomotion and feeding. This potentially creates a covariance such that genetic variance in larval growth may depend upon differences in larval swimming activity. The hypothesis that variation in larval growth may depend upon variation in swimming performance leads to two predictions: (1) if swimming speed varies among families larval growth rate will also vary among families and vice versa, and (2) variation in mean rates of larval growth and swimming will positively genetically covary. We tested these predictions by measuring growth rates and swimming speeds of over 1300 and 2000 larvae, respectively, from over 100 full-sib families of the marine snail Crepidula fornicata. Adult C. fornicata release veliger larvae that have an opposed band ciliary system that generates both feeding and locomotory currents. We detected significant variation among families in both larval growth rate and swimming speed. Broad sense heritabilities were similar in both cases and indicate that about one-third of the variation in larval growth and swimming speed may be genetic in origin. There was, however, no significant covariation between larval growth rate and swimming speed, indicating that the two traits are independent.