Reproduction in the Blacktail Rattlesnake, Crotalus Molossus (Serpentes: Viperidae)‪.‬

Abstract. -- Reproductive tissue was examined from 107 museum specimens of Crotalus molossus from Arizona, New Mexico, Texas and Mexico. Males follow a seasonal testicular cycle in which spermiogenesis occurs May-September. Sperm was present in the vasa deferentia from all months examined: April-May, July-September. The smallest spermiogenic male measured 576 mm SVL. Enlarged ovarian follicles were present in April, July, September-October. Oviductal eggs were present April and June. Developing young were present in July. Mean brood size (n = 25) including 15 records from Klauber (1972) was 5.2 [+ or -] 3.1 SD, range 2-13. The smallest reproductively active female (enlarged follicles) measured 653 mm SVL. Females appear to have a biennial reproductive cycle as has been reported for other rattlesnakes. Only 10/40 (25%) of female C. molossus were reproductively active. **********