During routine examination of fish collections from several southwest Texas streams, we noted an apparent dimorphism in the length of pectoral and dorsal fins in specimens of plains minnow Hybognathus placitus. Although sexual dimorphism in morphological characters other than color and the presence of nuptial tubercles is uncommon in minnows, sexual dimorphism in length of fins (longer in males) has been noted in some species. Here, we present an analysis of this dimorphism and test the hypothesis that plains minnow specimens with longer fins are males. We digitized 16 external morphological landmarks on 62 plains minnows ranging in total length from 7.1 to 8.4 cm. These landmarks were used to generate 21 external length measurements. We used discriminate function analysis to differentiate between males and females based upon 21 measurements. Preliminary identification of males and females was based on dorsal and pectoral fin characteristics. Of the 21 measurements, ten had significant (P < 0.05) discriminant coefficients. The first dorsal fin ray had the greatest loading (0.83) followed by the pectoral fin length (0.56). The male plains minnow predominantly have larger heads and peduncle regions whereas the females have larger abdomens. After analyses were complete we verified sex by evisceration of the gonads and histological inspection. Sixty-one of 62 fish were correctly sexed based on pectoral and dorsal fin characters.