Lachesis muta (Viperidae: Crotalinae) is distributed from the southern tropical regions of Brazil to the Atlantic and Pacific slopes of Costa Rica (Vial and Jiménez-Porras, 1967; Peters and Orejas-Miranda, 1970; Hoge and Romano-Hoge, 1981; Bolaños, 1982). Although Villa (1962) reported its presence in southern Nicaragua, there is very little information on this species in Nicaragua. Currently, Lachesis muta includes three subspecies: (a) L. m. rhombeata, present in the rainforest areas of the Atlantic regions of Brazil, from the State of Alagoas to Rio de Janeiro; (b) L. m. muta, distributed in the equatorial rainforest of Brazil, Guyanas, Venezuela, Trinidad, Peru, Ecuador, Colombia and Panama (Martínez and Bolaños, 1982); and (c) L. m. stenophrys, present in the tropical rainforest of Costa Rica and Panama. In Costa Rica, this subspecies is widely distributed along the Atlantic slopes, in the northern plains of San Carlos, Alajuela Province, and a peculiar population ranges from the region of Puerto Cortes, to Peninsula de Osa, Valle de Coto Brus and Valle de Coto Colorado. In Costa Rica, the central mountain range, with elevations of 2000-3000 in, constitutes a strong geographic barrier that separates Atlantic and Pacific populations of L. muta as well as other snakes (Taylor et al., 1974). In the serpentarium at the Instituto Clodomiro Picado we observed that specimens of L. muta from the Pacific region of Costa Rica display several features that clearly distinguish them fromthe typical pattern of L. m. stenophrys. Previously, Bolaños et al. (1978) described differences between these two populations in immunological characteristics of their venom. Recent morphological studies of the southeastern populations of L. muta from Costa Rica prompt us to conclude that it represents a new subspecies. In this work we describe this subspecies and compared it to other subspecies of L. muta, especially to L. m. stenophrys.
