This paper presents the results of plankton studies in the Peruvian upwelling region off Chimbote from November - December, 1977, during the Investigación Cooperativa de la Anchoveta y su Ecosistema (ICANE). Primary productivity, respiratory ETS activity, composition of particulate organic matter, and microplankton cell numbers were determined. Phytoplankton growth, and bacterial and ciliate carbon-uptake rates were computed from cell counts.Inshore waters were dominated by diatoms and were more productive than offshore waters which were dominated by dinoflagellates and ciliates. Particulate primary production averaged 5.26 ± 5.24 g C m-2d-1, and the POC standing stock was 7.75 ± 2.74 g C m-2 for the euphotic layer of 7 shelf stations. On the shelf, microplankton respiration rates were higher in plankton populations dominated by dinoflagellates and ciliates (47% of particulate primary production per 24 h) than those in diatom dominated populations (11%, respectively). The diatom populations, which were dominated by Chaetoceros species, varied in their ecophysiological properties (assimilation numbers, proportion of water soluble carbohydrate, and protein/nitrogen ratios). The relationships between these variations and growth conditions were investigated. A 40 h time-series station revealed patchiness which was superimposed on physiological changes of the plankters. Bacterial numbers of 2 × 106 cells/ml were found in the euphotic layer corresponding to approximately 17 μg C/l bacterial biomass (or 6% of the POC standing stock). Ciliate biomass (Lohmanniella oviformis was the dominating species) ranging from 2 to 9% of the POC standing stock were found even in diatom dominated populations. From a rough carbon balance for the euphotic layer it was deduced that in diatom dominated populations, 36-77% of particulate primary production was potentially available to adult anchoveta grazing. © 1982 IRL Press Ltd.