A cytogenetic analysis of two anuran species, Xenopus laevis laevis and Xenopus muelleri, was performed. The diploid number of chromosomes in both species was 2n = 36. The two karyotypes, arranged according to size of chromosome and centromere position, showed no pronounced differences. However, the chromosomes which had a secondary constriction were different. In X. laevis this constriction (nucleolar organizer) was found on one pair of acrocentric chromosomes only, the pair showing association in about 50% of the metaphases. In X. muelleri, on the other hand, two distinct pairs of submetacentric chromosomes contained secondary constrictions. Neither of these two chromosome pairs was morphologically similar to the constriction-bearing chromosomes of X. laevis. Association similar to that in X. laevis was found in one pair only, the one with a terminal constriction. In neither of the two species was there any evidence of sex-chromosome heteromorphism.