Copepods dominate the zooplankton, but surprisingly little is known of their tolerance to the increasing threat of hypoxia. We measured abundances of the calanoid copepod Calanus pacificus in relation to oxygen concentrations in the field and established its tolerance to low dissolved oxygen (DO) in the laboratory. In situ distributions of female C. pacificus were assessed with depth-stratified net sampling in Hood Canal, a seasonally hypoxic sub-estuary of Puget Sound, Washington. No clear avoidance of DO levels from supersaturated down to 2.0 mg DO L−1 was observed; DO levels <2 mg L−1 were rare in our sampling. In the lab, at 13°C female C. pacificus exhibited 100% 24-h survival at DO levels down to 1.7 mg DO L−1; below 1.5 mg DO L−1, survival sharply declined, with 25% survival at 1.2 mg L−1 and complete mortality within 1 h at 0.9 mg L−1. Calanus pacificus is one of the dominant copepods throughout much of the North Pacific and an important trophic link in the ecosystem. Our measurements indicate that they have a steep threshold in oxygen tolerance that is similar to those measured for other calanoid copepods and which could limit their habitat in years of severe oxygen depletion.