Among Mexico’s diverse herpetofauna, the rattlesnakes (Crotalus and Sistrurus) have intrigued both herpetologists and non-herpetologists alike for centuries. One group of rattlesnakes, however, has remained especially enigmatic. The long-tailed rattlesnake group includes three rare species (C. stejnegeri, C. lannomi, and C. ericsmithi) and is represented in museum collections by fewer than 15 specimens, most of which are C. stejnegeri (Campbell and Flores-Villela 2008; Campbell and Lamar 2004). Their unique body structure and unusually long tails have raised numerous questions regarding their natural history and ancestry. The Autlán Long-tailed Rattlesnake, C. lannomi, has been known only from a single specimen, collected in June 1966 by Joseph R. Lannom, Jr. from “1.8 miles west of the pass: Puerto Los Mazos, Jalisco” (Tanner 1966). Despite numerous attempts to locate additional specimens (Campbell and Flores-Villela 2008; Campbell and Lamar 2004), none have been reported since the initial description. Since 2004, we have searched for rattlesnakes in the western foothills of the Sierra Madre Occidental and associated ranges in Nayarit, Jalisco, and Colima. Over the course of several weeks in July 2008, five additional specimens of long-tailed rattlesnakes were found in the foothills of Colima, México. Based on features of lepidosis, this new material is referred to Crotalus lannomi. The snakes were found approximately 50 km SW of the type locality (Fig. 1). Herein, we provide detailed morphological descriptions of the new specimens, notes on natural history, comparisons with the other species of long-tailed rattlesnakes, and review the conservation status of the species.