Daphnia magna, Daphnia pulex, Hyalella azteca, and Gammarus lacustris were exposed to low dissolved oxygen concentrations in the laboratory. Acute and chronic exposures were conducted to develop data for use in the U.S. Environmental Protection Agency's (EPA's) water quality criteria document for dissolved oxygen. Daphnia magna had two 48-h LC50s (50% mortality) of 0.6 and 0.7 mg/L, a highest-adverse-effect concentration of 0.6, and a lowest-no-adverse-effect concentration of 0.9 mg/L O2 (based on reproduction). Daphnia pulex had a 48-h LC50 of 0.5, two 96-h LC50s of 0.4 and 0.7, a highest-adverse-effect concentration of 1.6, and a lowest-no-adverse-effect concentration of 2.1 mg/L O2 (based on reproduction). Hyallela azteca had 96-h and 30-d LC50s of < 0.3 mg/L, a highest-adverse-effect concentration of 1.2, and a lowest-no-adverse-effect concentration of > 1.2 (based on reproduction). Gammarus lacustris had two 7-d LC50s of < 0.2 mg/L, a highest-adverse-effect concentration of < 0.2, and a lowest-no-adverse-effect concentration of 0.1 mg/L O2 (based on survival). Lack of access to the water surface increased mortality. Nitrogen-stripped and vacuum-degassed test water produced the same mortality. Results of the study indicate that dissolved oxygen concentrations adequate to avoid impairment of fish production, as outlined in the EPA's water quality criteria document, should provide reasonable protection for these four species.