All the karyotypes of ten snake species of the family Viperidae from Costa Rica have a diploid number of 36 chromosomes, 16 macro- and 20 microchromosomes; females show an heteromorphism in the fourth pair. The karyological differences are restricted to the position of the centromere in the eighth chromosome pair and in the W-sex chromosome. The karyotypes of Bothrops asper, B. nigroviridis. B. schlegelii, B. nummifer, B. picadoi, B. nasutus and Crotalus durissus durissus are of an identical pattern, having a subtelocentric eighth pair and a submetacentric W-sex chromosome. The autosomal pattern of B. godmani and Lachesis muta stenophrys is similar to that of the above group, except that the W-sex chromosome is subelocentric. The karyotype pattern of B. lateralis is different from these in that it has a submetacentric eighth pair.