The karyological characters differentiating the Amphibia from most other Vertebrates are the tendency toward genome hypertrophy, the occurrence of wide interspecific differences in nuclear DNA amounts and - in contrast - the relative conservativeness in karyotype morphology exhibited by the ‘higher’ families from the three orders. On examination of the more advanced Urodeles (Plethodontids and Salamandrids) these characters seem to represent the outcome of selective pressures tending on the one hand to favour the accumulation and speedy divergence of new DNA fractions, specifically the middle repetitive ones, and on the other to maintain or reduce the number of adaptive linkage-groups acquired through evolution. The primitive Urodela (Cryptobranchoidea) display only the first two of the above three characters. Their karyotypes are in fact widely variable in chromosome number and shape at the intergeneric and occasionally interspecific levels; the most prominent trend, from the karyological viewpoint, seems to be towards a progressive reduction in number of acrocentrics and microchromosomes. Evidence so far is lacking as to the presence of peculiar fractions of genomic DNA in the microchromosomes from these Urodeles.