Age- and sex-specific grazing rates have been studied in specimens of Bithynia tentaculata from Oneida Lake, New York. Two artificial foods and Aufwuchs grown in Oneida Lake were used as grazing substrates. These foods were of different protein quality and organic content. Concomitant analyses of assimilation, respiration and nitrogen excretion allowed assessments of bioenergetic partitioning through the life cycle. Weight-specific hourly rates for ingestion, egestion, respiration and nitrogen ex- cretion were measured. These measurements provided data for calculation of assimila- tion and nonrespired assimilation. Weight-specific rates of food, carbon and nitrogen allocation were used to construct bioenergetic profiles for breeding females. Such data were coupled with laboratory fecundity and growth data and were used to examine hypotheses of age-specific and trophic-limited reproduction in the iteroparous popula- tion of Bithynia studied. Older females have a higher reproductive effort than younger regardless of trophic constraints. Further, protein may be a limiting factor in egg production. As predicted from hypotheses on age-specific reproduction, younger females always have a propor- tionately lower reproductive effort than older females (measured as net reproductive biomass or as reproductive biomass as a fraction of nonrespired assimilation). Trophic conditions influence patterns of fecundity. Consistent with theory on reproductive fac- tors, younger females on low protein food curb their reproductive effort in favor of somatic growth and maintenance.