The location, size, and geography of California, combined with extensive knowledge of successful and failed fish invasions, provide an unusual opportunity to test predictors of invasion success. Our analyses show that different characteristics of alien fishes are important at different stages of the invasion process. We found no set of characters that predicted success for all fish invasions, although some characters increase the probability of success. The factors that best predict invasion success are (a) a history of successful establishment outside the species\textquoteright native range; (b) characters that promote success at multiple stages of the invasion process (e.g., high physiological tolerance); (c) invaded habitat that more or less matches the alien\textquoterights native habitat; (d) high fish species richness, including other alien fishes; and (e) propagule size exceeding 100 individuals. The difficulty of predicting the invasion success of alien species points to the need to allow only introductions that have proved to be nonharmful and to take quick action to prevent the spread of new invaders.