Biomass, species composition and production of the planktonic community were investigated during the rainy season in May and June 1999 outside Zanzibar Island, Tanzania. In general, the plankton biomass of different organisms was uniform between depths as well as over time. The integrated water column primary production ranged from 204 to 4142 mg C m–2 day–1. Bacterial production varied between 10 and 72 mg C m–2 day–1, comprising ~5% of the total bacterial standing stock. The data obtained from these experiments are summarized in a carbon budget. At the most 77% of the total primary production channelled through the heterotrophic flagellates, ciliates and heterotrophic dinoflagellates to higher trophic levels. Of the estimated carbon demand for mesozooplankton, 28% could potentially be met by ciliates and heterotrophic dinoflagellates.