Salinity stress is a major factor determining the abundance and distribution patterns of marine organisms. This is particularly relevant for organisms that live near freshwater sources such rivers and streams, like the ghost shrimp Neotrypaea uncinata. In addition branchial chamber of this species is parasitized by the bopyrid isopod Ionella agassizi. In consequence, this species is exposed at two stress sources, osmotic due to salinity variations and respiratory for the parasite effects in its respiratory physiology. We evaluated the effects of both salinity and parasitism on the concentration of respiratory molecules and the regulation of the acid-base equilibrium. We measured the hemolymphatic pH, PCO(2) and the concentrations of Na(+), K(+), Cl(-), Ca(2+), HCO(3)(-), in parasitized and non-parasitized individuals of Neotrypaea uncinata exposed to different hypo-osmotics salinities (20, 25, 30 and 35 psu). Results showed that all the individuals, parasitized and non-parasitized, exhibited an osmoconformer behaviour along almost the entire salinity gradient, without effect in the acid base equilibrium in this species. In general, the parasite did not significantly affect the measured variables, this could indicate the presence of compensatory physiological mechanisms.