There are two aspects to sexual selection (Darwin, 1871). Intra-sexual selection focuses on the consequences of competition between members of one sex (usually males) for access to the other sex; whereas epigamic selection or female choice focuses on the consequences of mate preferences of one sex (usually females) with regard to the other sex. Agonistic encounters between males in which differences in body size would influence the outcome have often been cited as a primary cause of sexual dimorphism in body size (Selander, 1972). In such encounters larger males are more likely to win and thereby exclude smaller males from breeding. As variability in male reproductive success increases, as in polygynous species, the degree of sexual dimorphism would be accentuated. Such correlations have been reported for a number of vertebrate groups (see Lande, 1980 for a summary). This explanation is incomplete, however, since it fails to address the question of what determines or limits female size. Darwin (1871) recognized this weakness, and noted that sexual dimorphism in fish was particularly troublesome to explain since "in many kinds of fishes the males fight together, it is surprising that they have not become larger and stronger than the females through the effects of sexual selection" (p. 332). He speculated that females may often be larger than males because larger females could produce more eggs and presumably the selective advantages of large size in females outweighed the selective advantage of large size in males. Consideration of female choice allows the question of sexual dimorphism to be rephrased as follows: In species where females choose among prospective mates, why should a female choose to mate with a male who is larger (or smaller) than herself? The answer to this question determines whether males or females are the larger sex. The answer may be a proximal one: males of all ages are larger than females (as is frequently the case for birds [e.g., Holm, 1973]), or males mature later and at a larger size than females (as is the case for most polygynous birds and mammals [Wiley, 1974]). Yet this response does not address the question of how such differences, in growth rates or age at sexual maturation, arose. Fishes seem a particularly useful group for evaluation of the evolution of sexual dimorphism. Situations in which males are the larger sex are not uncommon, nor are instances where females are the larger sex (Breder and Rosen, 1966). The mottled sculpin (Cottus bairdi) is sexually dimorphic in body size, males being the larger sex (Fig. 1). In mottled sculpins growth is indeterminate and differences in size reflect differences in age (Downhower, unpubl.). Sexual differences in age at first reproduction are termed sexual bimaturism (Wiley, 1974); however, we have chosen not to use that term in our discussion because we feel the evidence indicates that size per se is the crucial issue. Here we describe the nature of female choice in this species and its relationship to sexual dimorphism in body size.