Analyses of skeletal and external morphology of spinner dolphins killed in to the yellowfin tuna purse-seine fishery in the eastern tropical Pacific led to the description of two subspecies of spinners from this region, the eastern spinnen dolphin and the Central American spinner dolphin (Perrin 1990). However, when we examined lengths of spinner dolphins taken from vertical aerial photographs from the same area, we found three unique morphotypes. Two of these forms correspond, at least in average length and distribution, to the existing subspecies. The third form is intermediate in length between the two recognized subspecies and is found along the edge of the continental shelf north of Cabo Corrientes, Mexico. We provisionally call this form the “Tres Marias spinner dolphin.” Our results demonstrate the value of a mix of fishery and fishery-independent data in studies of stock structure of impacted species.