Population growth efficiencies (GE) of 22 laboratory populations of Hydra pseudoligactis fed Artemia sp. nauplii were determined. Maximum growth efficiencies of ca. 37% were observed for ingestion rates greater than 0.5 cal each day per calorie of hydra. The instantaneous growth rate increased continuously with increased ingestion above the minimum of ca. 0.25 cal each day per calorie of hydra. The maximum calculated net efficiency was ca. 55%. Although GE was constant above ingestion rates of 0.5, net efficiency and growth rates continued to increase with increasing rates of ingestion.