Hybridization between two sucker species, Razorback Sucker Xyrauchen texanus and Flannelmouth Sucker Catostomus latipinnis, historically has occurred but has been facilitated by anthropogenic habitat alteration. The ability to differentiate Razorback Suckers from Flannelmouth Suckers and hybrids is essential for the conservation and management of the endangered Razorback Sucker. The purpose of our study was to investigate allometry-that is, how shape changes with length-of Razorback Sucker, Flannelmouth Sucker, and their hybrids. Razorback Sucker and Flannelmouth Sucker were artificially spawned to produce four progenies: Razorback Sucker, Flannelmouth Sucker, female Razorback Sucker × male Flannelmouth Sucker and female Flannelmouth Sucker × male Razorback Sucker. We also used hatchery-reared Razorback Sucker and wild-captured Flannelmouth Sucker to fill the size-class gaps in our data. We conducted geometric morphometric analyses and used conditional inference trees to determine the length at which body shapes begin to differentiate among these four fish types. We used piecewise linear models to understand the allometry of each fish type and to evaluate the size at which body shape begins to differ from the juvenile form. Although we found statistical shape differences between the two hybrids, they would be difficult to detect with the naked eye. The results of our study showed that fish that are smaller than 140 mm total length (TL) are difficult to differentiate using shape alone. With the information provided in this study, field biologists should be cautious when identifying suckers smaller than 140 mm TL in the Colorado River basin.The Colorado River basin is a prime example of a southwestern riverine ecosystem that has been heavily altered by humans (Miller 1961;Gutermuth et al. 1994;Albrecht et al. 2010). The basin encompasses seven U.S. states and two Mexican states. After the construction of Glen Canyon Dam, the Colorado River in the Grand