The karyotypes have been determined of 16 of the 32 species of the genus Varanus, including animals from Africa, Israel, Malaya and Australia. A constant chromosome number of 2n = 40 was observed. The karyotype is divided into eight pairs of large chromosomes and 12 pairs of microchromosomes. A series of chromosomal rearrangements have become established in both size groups of the karyotype and are restricted to centromere shifts, probably caused by pericentric inversion. Species could be placed in one of six distinct karyotype groups which are differentiated by these rearrangements and whose grouping does not always correspond with the current taxonomy. An unusual sex chromosome system of the ZZjZW type was present in a number of the species examined.

A chromosomal, analysis of the monitor Varanus acanthurus Boulenger has been made using G-and C--banding and silver-staining techniques. This species has two cytotypes, one of which has a pericentric-inversion polymorphism, whereas the other is chromosomally monomorphic. A ZZ,.' ZW sex-chromosome system is also present in both cytotypes of this species. The banding patterns of these mechanisms are described and their evolution is discussed.

The taxonomic relationships of frogs from the genus Cyclorana are uncertain, for they share similarities with both the Leptodactylidae and the Hylidae. Karyotypic comparisons were made between seven species of Cyclorana and three species from the leptodactylid genera Notaden, Neobatrachus and Heleioporus. All Cyclorana species have 2n = 26, whereas the other species have 2n = 24. The position and type of secondary constrictions subdivide the species of Cyclorana into three groups. The similarity of chromosome morphology in these three groups with that of hylids suggests a close phylogenetic relationship. The major chromosome differences between Cyclorana and representatives from the leptodactylid genera suggests a fundamental dichotomy masked by adaptation to a fossorial mode of life.