The larval and pupal stages of Chironomus riparius are benthic dwellers which ingest sediment, and are commonly prey. In an environment with elevated selenium concentrations, they can accumulate it and transfer it to higher trophic level organisms upon being ingested. This study was conducted to: 1. Develop a method for performing acute toxicity tests using Chironomus riparius. 2. Establish relationships between dry mass and selenium content, and between dry mass and selenium body burden. 3. Determine whether bioaccumulation or bioconcentration contributed more to the selenium content and body burden of individual larvae. 4. Determine selenium concentrations which were acutely and chronically toxic to C. riparius, and whether previous exposure changed the acutely toxic concentration. 5. Evaluate adequacy of national surface water selenium criterion with respect to these data. In acute toxicity tests, 1 larva per 4 ml test solution was an acceptable organism density, and test solutions were successfully renewed by siphoning and refilling the test chamber. Regression showed that larval selenium content was linearly dependent on dry mass within instar, and body burden was negatively exponentially dependent on dry mass. Predicted body burdens were linearly dependent on dissolved selenium concentration. This indicated that bioconcentration was more important than bioaccumulation in determining body burdens. C. riparius readily acclimated after prolonged exposure to dissolved selenium, but became slightly more sensitive to selenium after prolonged exposure to substrate-adsorbed selenium. The national water quality selenium criteria may not protect against biomagnification in all ecosystems, and should be based on some environmental chemical factors. / Master of Science