The mitotic chromosomes, nucleolus organizer regions (NORs), C-banding patternand nuclear DNA content ofDiplomystes mesembrinuswere studied. The karyotype, with2n=56chromosomes (22m+24sm+6st+4a), has a high chromosome arm number (NF = 102),one chromosome pair with NORs, and a very small amount of heterochromatln. The NORbearing arm is entirely heterochromatic and exhibits a marked size polymorphism. The diploid DNA content detected in erythrocyte nuclei of D.mesembrinuswas 2.57 ± 0.15 pg/nucleus. The chromosome evolution in Siluriformes is discussed on the basis of available cytogenetic data and it is proposed that 2n=56 is synapomorphic for the order.