SUMMARY -In fish cytogenetics the methods of chromosome preparations still need to be improved in order to obtain good metaphase figures for banding technics and more accurate analysis. In this paper we describe a very fast and simples culture method from solid fish tissues. Results of its application to different species as well as about 5-BrdU incorporation, both for chromosome banding and sister chromatid differentiation are reported. The data emphasized the advantage of this short term culture method wich provide excellent chromosome preparations suitables for basic and applied cytogenetic studies.

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We studied the karyotypes of Hassar cf. orestis and an undescribed Hassar species from the Jarí River and Opsodoras ternetzi, H. orestis and Platydoras cf. costatus from the Xingú River, all with 2n = 58. Constitutive heterochromatin is located in the centromere in most metacentric pairs; in some chromosomes this banding is not present, or it is located on the whole chromosome arm or in the distal regions. The NOR is located on a single biarmed pair at a distal region of the short arm in H. cf. orestis, H. orestis and P. cf. costatus at a distal region of the long arm in O. ternetzi and at a proximal region of the long arm in the Hassar species. In all species (except for Hassar sp.) the CMA3 analysis revealed a rich G-C region coincident with the NOR. Probably inversions occurred in the NOR chromosome during the chromosomal differentiation of the Doradidae species here described.