A cytological analysis of the anuran species Xenopus tropicalis was performed. The diploid chromosome number is 2n = 20, whereas two other Xenopus species previously analyzed, laevis and muelleri, have 2n = 36. In X. tropicalis the morphology of the various chromosome groups differs from the other two species. The karyotype arranged according to size of chromosomes and centromere position falls into six morphological groups of chromosomes. Two distinct pairs bear secondary constrictions. Neither of these is similar to the constriction-bearing chromosomes of X. laevis (nucleolar organizer) or X. muelleri. The aspect of their association is also different. There is no evidence for sex-chromosome hetero-morphism. The karyotype analysis suggests that X. tropicalis is systematically quite separate from the two other species mentioned above.