Sodium balance and mortality rates were examined in five species of adult waterboatmen (Hemiptera: Corixidae) exposed to neutral and low pH waters. The five species were chosen to reflect a wide range of pH conditions in waters where they naturally occur. Cenocorixa bifida and C. expleta normally inhabit high pH waters, whereas C. blaisdelli and Hesperocorixa atopodonta can be found most often at neutral pH. Sigara omani occur in acidic waters. Haemolymph [Na], whole-body [Na], and sodium influx rates were recorded during exposures of 6-9 days at pH 7.0, 4.5, and 3.0. C. blaisdelli and H. atopodonta were studied throughout the year (spring, summer, and fall) . C. bifida and C. expleta were examined in the fall, while S. omani were tested in the spring. Overall, these corixids appeared to be tolerant of short-term exposure to low pH. Mortality for all species remained below 50% in both neutral and acidic pH levels throughout the year. Additionally, differences in haemolymph [Na], whole-body [Na], and sodium influx rates from pH 7.0 to either pH 4.5 or 3.0 were rarely significant. However, both inter- and intraspecific variation in sodium balance over the range of pH levels tested were apparent. Most notably, C. blaisdelli and H. atopodonta exhibited their highest concentrations of haemolymph and whole-body Na in the summer, under both natural conditions and in the laboratory at all pH levels. For C. blaisdelli, summer was also the time of highest mortality, with mortality increasing as pH was lowered. The observed correlation between high haemolymph/whole-body [Na] and high mortality in the summer appears to result from a relatively large decrease in haemolymph and whole-body [Na] when bugs were exposed to pH 3.0, at a time when these values were initially high. Fall was the season of lowest haemolymph/whole-body [Na], and also of lowest mortality, for both C. blaisdelli and H. atopodonta. Interspecific variation in the ability to tolerate low pH did not appear to reflect variation in the pH of water these bugs normally inhabit. Of the five species tested, H. atopodonta appeared best able to maintain internal homeostasis under acidic conditions, despite being common to neutral waters. Haemolymph [Na], whole-body [Na], and sodium influx rates did not change significantly from pH 7.0 to pH 3.0 in any season. Furthermore, C. blaisdelli, which was collected from the same pond as H. atopodonta, seemed to be least able to regulate internal milieu in low pH waters. In C. blaisdelli, exposure to pH 3.0 usually resulted in decreased haemolymph and whole-body [Na], relative to values recorded in those individuals exposed to pH 7.0. The difference in the size of these two species (H. atopodonta is approximately twice that of C. blaisdelli) may account for the observed variations in their respective sodium balance at low pH. / Science, Faculty of / Zoology, Department of / Graduate