Karyotypes and nuclear DNA content were studied in 11 species of the genus Corydoras from rivers in South America: C. sp. from Caripi river 2n=60, C. cf. simulatus 2n=62, C. simulatus 2n=62, C. reticulatus 2n=74, C. sp. from Galheiro river 2n=84, C. aff. punctatus from Negro river 2n=102, C. flaveolus 2n=58, C. arcuatus 2n=46, C. trilineatus 2n=46, C. schwartzi 2n=46, and C. metae 2n=92. Extensive chromosome diversity and differences in DNA content were detected among species. The high variability in chromosome counts was not exclusively related to chromosomal structural rearrangements, but also to large changes in DNA content. Species could be grouped using their shared cytogenetic characteristics, suggesting that within the genus Corydoras different groups of species followed distinct evolutionary trends. Chromosomal rearrangements in Corydoras are, apparently, more frequent that morphological modifications, so cytogenetic data may be very useful for species delimitation and for the understanding of interrelationships among species.