Caridean shrimps are unique among decapod crustaceans in showing a great diversity of sexual systems, including gonochorism, protandry, protandry with primary females and simultaneous hermaphroditism. Crangon franciscorum (Stimpson, 1859) (Crangonidae), an ecologically and economically significant species from the Pacific coast of North America, has been assumed to be gonochoristic. Earlier population studies concluded that average lifespan is longer in females than in males, and that the latter die in or emigrate from estuaries after mating. This was believed to explain a recurrently observed “shrinking” of average male size starting during the fall of the first year of life. We investigated an alternative hypothesis according to which this species is a protandric hermaphorodite, and disappearance of large males from the population reflects sex change rather than death or emigration. We present several pieces of evidence in support of the alternative hypothesis: (i) ovarian development in males undergoing sex change, (ii) presence of atrophied vas deferens in secondary females, and (iii) observation of sex change in individuals kept in captivity. Our results are supported by histological study of the gonads, careful description of secondary sexual characters, and monthly sampling of an estuarine population (Grays Harbor, Washington). Tracking an identifiable year-class (1980) and combining field and laboratory data allowed us to assemble a life history schedule, including growth and a calendar of significant reproductive events. We discuss the implications of our results for the interpretation of survey data and studies on population dynamics.