Aside from the type series of _Crotalus pusillus_ from Cerro Tancítaro and one specimen from Carapan referred to the species by Klauber (1952:38), there are fourteen specimens from the Sierra de Coalcomán. These specimens (UMMZ 112566-7, 118591-9, 118601, 121512-3) are like _Crotalus pusillus_ from Cerro Tancítaro in having the prefrontals paired, a black proximal rattle, and the underside of the tail black. The prefrontals are bordered posteriorly by one scale in two specimens, by two scales in three specimens, and by three scales in the other nine. The snakes from the Sierra de Coalcomán have 40 to 46 (42) dorsal body blotches. Ten males have 150-158 (154.4) ventrals and 29-33 (31.0) caudals; two females have 157 and 160 ventrals, and 25 and 27 caudals. The largest specimen is a male having a body length of 545 mm. and a tail length of 63 mm. The only noticeable difference between the specimens from the Sierra de Coalcomán and the topotypic series is that the latter have fewer dorsal blotches; the range of variation is 33 to 46 (39.8). Most specimens of this species have a grayish brown dorsum and dark brown dorsal blotches. Two specimens from Dos Aguas (UMMZ 118596 and 118599) are pale brown above and have indistinct blotches. One specimen from Dos Aguas regurgitated a large _Gerrhonotus imbricatus imbricatus_; of two others from the same locality, one regurgitated a _Sceloporus bulleri_ and an _Eptesicus fuscus_. The latter specimen was collected at the entrance of a small cave, where it probably had captured the bat. In the Cordillera Volcánica _Crotalus pusillus_ has been obtained in pine-oak forest at elevations between 1550 and 1800 meters. In the Sierra de Coalcomán two specimens were taken in pine forest at an elevation of 2300 meters; ten other were found beneath rocks and logs in pine-oak forest at an elevation of 2100 meters. ~Crotalus triseriatus aquilus~ Klauber _Crotalus triseriatus aquilus_ Klauber, Bull. Zool. Soc. San Diego, 26:24, August 8, 1952.--Alvarez, San Luis Potosí, México. Morelia (10); Tacícuaro (2). I am following Klauber (1952) in assigning some of the specimens of this species from Michoacán to the subspecies _aquilus_ and others to _C. t. triseriatus_. The distinguishing characters of these subspecies are given by Klauber (1952:28). On the basis of the few localities from which the species is known in Michoacán it seems as though _C. t. aquilus_ inhabits the open grassy areas on the Mexican Plateau and the associated open pine-oak or oak-bunch grass habitats to the north and east of the Cordillera Volcánica. _Crotalus triseriatus aquilus_ has been collected at elevations from 1600 to 2000 meters in Michoacán. ~Crotalus triseriatus triseriatus~ (Wagler) _Uropsophus triseriatus_ Wagler, Natürliches System der Amphibien, p. 176, 1830.--México. (Probably Mexico City.) _Crotalus triseriatus triseriatus_, Klauber, Bull. Zool. Soc. San Diego, 26:19, August 8, 1952.