The freshwater turtle, Phrynops geoffroanus lives in the water, being capable of submergence lasting many hours or days. Since these turtles can survive to anoxic or hypoxic conditions, that ability prompted us to investigate their hemoglobins, from the standpoint of their functional properties and characterization of their aggregation state. Blood samples were processed and partially purified hemoglobins (by gel filtration using Sephacryl S-100 HR) were subsequently analyzed concerning O₂-binding properties and allosteric control by tonometry. We studied pH dependence of the oxygen affinity (Bohr Effect), effects of temperature and phosphate binding. Since the hemoglobins from P. geoffroanus appear to easily aggregate, we performed polymerization analysis using gel filtration chromatography and eletrophoretic mobility. The functional data showed an alkaline Bohr effect (DH+= -0.18) and modulation by phosphates, that increase the response to proton binding (DH+= -0.57). These hemoglobins showed cooperative oxygen binding under all experimental conditions and in the whole tested pH range. Oxygen binding and cooperativity were temperature dependent, a favorable process ensured by a negative variation of enthalpy. The hemoglobins displayed a peculiar behavior that would suggest two sites for phosphate binding: an initial decrease of the oxygen affinity revealed a stabilization of the T-state of the hemoglobin molecule for the first part of the titration curve, and a high-affinity site that unusually stabilized the R-state structure, thus increasing the oxygen affinity in the subsequent part of the curve. We observed the presence of polymers presumably due to intermolecular disulfide bonds between tetramers. As proposed previously by Reischl et al. (1984), SH rich hemoglobins could act as a protection against reactive oxygen species during hypoxia and re-oxygenation. Financial Support: CAPES (ALF), FAPESP (03/00085-3) and CNPq (GOBR).