Sexual dimorphism of the upper jaw in Gillichthys mirabilis by C. Ben Crabtree. Bull. Southern California Acad. Sci., 84(2):96-103, 1985. Gillichthys mirabilis, the longjaw mudsucker, is easily recognized because of the posteriorly elongated maxillaries. Although expressed in both males and females, this feature is more conspicuous in the males. Simple linear regressions of upper jaw length indicate statistically significant differences in the development of this feature between the sexes. Growth of the maxillaries is relatively constant in young individuals, but the rate of growth increases with maturation. In both sexes, the rate of increase in the length of the upper jaw accelerates with age, but to a greater degree in the males, resulting in a sexually dimorphic character. Quantitative description of the ontogenetic development of this feature is best described by a nonlinear power function. Based on this investigation, the description of allometric growth may be more accurately addressed using nonlinear regression analysis rather than the standard linear or log-linear regression procedures. The dimorphism in this feature has little apparent relevance to diet. Although the females analyzed contained a higher proportion of the small prey item size class than the males, differences between the sexes were nonsignificant. The primary function of this feature appears to be its use in sexual behavior: display and territorial defense.