The present papers deals with the interspecific variations in the circadian rhythms of RBC counts and Hb contents in different species of the genus Mystus. The maximum values of RBC counts and Hb contents were recorded between 9.0 to 11.0 PM. in M. vittatus and between 3.0 to 7.0 PM. in M. cavasius while the M. bleekri behaved differently showing two peaks, first at 9.00 A.M. and next at 1.00 A.M. (night). The relevance and importance of such rhythms in aquaculture have been discussed.