Electrophoretic patterns of hemoglobins, hematological parameters, red-blood-cell phosphates, and whole-blood Bohr effect of Prochilodus cf. nigricans were studied during three different periods of the year. The main erythrocytic phosphates were ATP and GTP. The GTP/ATP ratio showed a significant diurnal variation suggesting regulation of NTP levels in response to daily variations of environmental and/or biological conditions. Prochilodus cf. nigricans showed four anodal Hb fractions in all studied individuals. However, differences in the relative Hb concentrations were determined at different times of the year. No significant hematological differences were detected during the study period, except for hematocrit values. Whole-blood Bohr coefficient was high in July and November, a time when water oxygen concentration was quite low.