Abstract: An improved BrdU-Hoechst-Giemsa method for revealing BrdU-replication bands in fish chromosomes was further modified and used for karyotypic analyses of Myloph aryugodon piceus and Ctenopharyngodon idellus. One important step when using this method is the timing of cell cycle of the fish studied. BrdU, AMD, Hoechst 33258 and EB, AO, especially the first three base-specific DNA-binding chemicals, were found to have effects on inhibiting condensation of fish chromosomes. The problem about the mechanism of BrdU-replication banding in fish chromosomes is briefly discussed.