1. Ecologists often measure the biomass and productivity of organisms to understand the importance of populations and communities in the flow of energy through ecosystems. Despite the central role of such studies in the advancement of freshwater ecology, there has been little effort to incorporate parasites into studies of freshwater energy flow. This omission is particularly important considering the roles that parasites sometimes play in shaping community structure and ecosystem processes. 2. Using quantitative surveys and dissections of over 1600 aquatic invertebrate and amphibian hosts, we calculated the ecosystem-level biomass and productivity of trematode parasites alongside the biomass of free-living aquatic organisms in three freshwater ponds in California, USA. 3. Snails and amphibian larvae, which are both important intermediate trematode hosts, dominated the dry biomass of free-living organisms across ponds (snails = 3.2 g m -2 ; amphibians = 3.1 g m -2 ). An average of 33.5% of mature snails were infected with one of six trematode taxa, amounting to a density of 13 infected snails m -2 of pond substrate. Between 18% and 33% of the combined host and parasite biomass within each infected snail consisted of larval trematode tissue, which collectively accounted for 87% of the total trematode biomass within the three ponds. Mid-summer trematode dry biomass averaged 0.10 g m -2 , which was equal to or greater than that of the most abundant insect orders (coleoptera = 0.10 g m -2 , odonata = 0.08 g m -2 , hemiptera = 0.07 g m -2 and ephemeroptera = 0.03 g m -2 ). 4. On average, each trematode taxon produced between 14 and 1660 free-swimming larvae (cercariae) infected snail -1 24 h -1 in mid-summer. Given that infected snails release cercariae for 3–4 months a year, the pond trematode communities produced an average of 153 mg m -2 yr -1 of dry cercarial biomass (range = 70–220 mg m -2 yr -1 ). 5. Our results suggest that a significant amount of energy moves through trematode parasites in freshwater pond ecosystems, and that their contributions to ecosystem energetics may exceed those of many free-living taxa known to play key roles in structuring aquatic communities.