The carbocyanine dye Cyan betaiPr selectively forms J-aggregates on the adenine-thymine sequences of DNA. The fluorescence quantum yield values of Cyan betaiPr J-aggregates formed on poly(dA)-poly(dT) polynucleotide were determined for the aggregates of different degree of order and for different aggregate absorption bands. It was established that the J-aggregate quantum yield values are near to that of the dye monomer molecules fixed on poly(dA)-poly(dT). The ordering of the aggregate structure leads to the increase of J-aggregate quantum yield and to the decrease in the efficiency of the transition from the upper aggregate exciton zone to the lower one. The electronic transition processes in Cyan betaiPr J-aggregates were discussed.