Various aspects of the biology of Hyridella menziesi were studied to obtain an energy budget so that its role in the Waikato River could be elucidated. Respiration rates were found to increase with both temperature and animal size. The Q₁₀ ranged from 1.1 to 8 °c to 2.6 at 27.5 °c. Weight dependent respiration increased exponentially with weight (in g. dry weight), the relationship being R = 417 W ⁰·⁴¹⁰ μg 0₂.hr⁻¹ at ambient temperatures (18.5 to 24.6 °c). The water pumping rate (P) increased with animal size (P = 3.8 w⁰·²⁷ ℓ.hr⁻¹), but filtering rates were variable, being affected by the particle retention efficiency of the gill. Retention efficiencies decreased in larger animals, a result of increasing interfilament distances which were evident from electron micrographs. Filtration rates using Chlorella showed no variation with animal size or food concentration (FC cells. ml⁻¹ ) , the mean being 1.02 ℓ.hr⁻¹, but the proportion of food ingested did depend on the amount available (ingestion ratio = 2.20 FC ⁻⁰·²²) and assimilation efficiencies (AE %), obtained by the dual isotope method of Calow and Fletcher, decreased as the feeding rate increased (AE 272 FR⁻⁰·¹²⁸, FR cells.hr⁻¹). Calculated energy budgets were positive but cell numbers in the Waikato River fell below that required for maintenance by animals of 0.05 g dry weight and larger during the winter months of July through September. The expected energy fluxes derived from river-bed transect and budget data were higher than those for other bivalves, but similar to that of Potamopyrgus antipodarum which is also found in the Waikato River, and is indicative of the intermediary role played by Hyridella of providing detritus for the benthos and speeding nutrient cycling.