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No significant differences in oxyhemoglobin affinity, or electrophoretic patterns of hemoglobin were found in 18 bull sharks collected in selected regions of Lake Nicaragua, the Rio San Juan, and the Caribbean Sea. The half-saturation of hemoglobin with oxygen (P50) was 11 and 17 mm Hg at pH 7.4 and 6.8, respectively (25° C, 3% hemoglobin solution, potassium phosphate buffer, 0.3 ionic strength). Electrophoresis resolved the hemoglobin into a minor and a major band. P1animetry of densitometric recordings showed that the major band constituted 54% of the total hemoglobin; the minor band, 46%. On the basis of these hemoglobin studies, no subspeciation of bull sharks in Lake Nicaragua was identified, although marine bull sharks have free access to the lake and have been there, at least, since 1535; the synonymy of Carcharhinus nicaraguensis with C. leucas was confirmed.