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In northern California rivers, rapid population growth of the chironomid Pseudochironomus richardsoni is associated with macroalgal blooms and with the warm temperatures and abundance of diatoms found in algal mats. Radioisotope techniques, including the dual-label method, were used to measure larval ingestion and digestion of diatoms in the laboratory The epiphytic diatoms used in these studies had a low C:N ratio (6.4) and organic matter content (31%) compared to other potential food resources of larvae in nature Mean individual ingestion rates increased 6-fold, from 0.0443 mg/d to 0.265 mg/d (dry mass), over a temperature range from 50 to 30 degreesC. Ingestion increased most rapidly between 5 degrees and 15 degreesC but showed a weak relation to temperature from 15 degrees to 30 degreesC. in contrast to ingestion rate, absorption efficiency of larvae was independent of temperature over a 20 degreesC range and averaged 32%. This result was surprising because absorption efficiency should decrease when gut passage time is reduced. These results suggest that there are compensatory changes in digestive processing leg, increased enzyme activity) that allow larvae to maintain constant absorption efficiency. This adaptation would benefit eurythermal consumers because the rate of absorption of energy and nutrients would also increase with temperature to help meet higher metabolic demands.