Attempts have been made to define the relationships among the South American camelids, the guanaco, llama, alpaca, and vicuna, by comparing the morphology of their incisors. The alpaca has been reported to have an incisor morphology similar to the vicuna, lacking enamel on the lingual surface. The llama and guanaco are said to have enamel on both the labial and lingual surface of their incisor teeth. These comparisons have been based on gross morphological observations and not on histologic analysis. This study was undertaken to determine whether or not alpaca teeth have enamel on the lingual surface. The cross-sectional histologic anatomy of the incisor teeth was compared in two closely related South American camelid species, the llama (Lama glama), and the alpaca (Lama pacos). Thick sections (300 ยตm) and scanning electron microscopy were the techniques utilized. The mandibular first, second, and third incisors were examined in four llamas and five alpacas. A substantial layer of enamel was present on all surfaces of all llama incisors. The enamel layer on the labial surface of the alpaca incisors closely resembled that found in the llama. The enamel layer on the lingual surface of the alpaca incisors, although greatly reduced, was distinctly present. Alpacas may be more closely related to guanacos and llamas than to vicunas. A histologic study of vicuna incisors would help to better define the relationships of the four camelids. Anat Rec. 249:441-449,