We provide the 1st documented accounts of the Mexican endemic rattlesnake Crotalus aquilus from the state of México. The new records extend the known distribution of the species into a region where it may be sympatric with the superficially similar C. triseriatus. Because these taxa have previously been subject to some taxonomic confusion, we performed a preliminary morphological comparison using individuals of both species obtained from proximal localities. Our analyses support the supposition that these taxa are morphologically distinct. The new localities for C. aquilus are situated in high valleys that have been extensively modified by human settlement and agriculture.