The role of bacteria and protozoa in the food web of Lake Kinneret was examined for 14 seasons over a 4-yr period (1989-1992) using a mass-balanced carbon flux model and network analysis. These microorganisms supplied nearly half of the carbon requirements of metazoan zooplankton grazers during the late winter-spring Peridinium bloom, when the lake was in its most eutrophic phase. The level of primary productivity was not seen to have a significant effect on the relative amounts of carbon passing from bacteria and protozoa to higher trophic levels. Rather, this depended on the proportion of photosynthetically fixed carbon that originated from 'inedible' net-phytoplankton and the efficiency with which bacterial carbon was transferred to metazoans. The importance of the microbial loop as a carbon source to higher trophic levels in Lake Kinneret was a result of the often high levels of inedible net-phytoplankton (mostly Peridinium gatunense). The higher levels also occurred because bacterial production was transferred to metazoans in a relatively efficient one- or two-step process. These results indicate that even in eutrophic environments, bacteria and protozoa can supply significant amounts of carbon to metazoans.