Ultraphytoplankton (<5 microns) and protozoan standing stocks, as well as their trophic interactions were investigated during five cruises to environments of different trophic status. ultraphytoplankton comprised approximately 60% of phytoplankton biomass during the oligo- to mesotrophic ne monsoon in the arabian sea, but only 11% in the southern red sea. as discriminated by flow cytometry, the ultraphytoplankton community was composed of the prokaryote genera prochlorococcus and synechococcus as well as at least two eukaryotic ultraplankton groups. the latter two and synechococcus dominated in terms of biomass. the eukaryotes became more prominent at increasing trophic status. prochlorococcus reached high abundances only at oligotrophic stations, and decreased to very low numbers at eutrophic stations. from these observations, together with additional references from the literature, it is speculated that prochlorococcus is a true obligate open ocean organism, highly adjusted to regenerating systems. in the oligotrophic gotland sea (baltic proper), ultraphytoplankton contributed to about 35% of phytoplankton biomass. synechococcus (up to 812,000 cm-3) and four eukaryotic ultraphytoplankton types were distinguished by flow cytometry. within the ultraphytoplankton community, biomass was dominated by the eukaryotic algae. in the meso- to oligotrophic open pomeranian bay, ultraphytoplankton <5 microns contributed to approximately 60% of phytoplankton biomass, whereas this share was reduced to 45% in the eutrophic river plume. in the pomeranian bay, up to 7 ultraphytoplankton groups were discriminated by flow cytometry and epifluorescence microscopy. next to synechococcus (up to 1,500,000 cm-3), three unidentified eukaryotic algae and two cryptophyte species comprised the ultraphytoplankton. biomass in the open bay water was dominated by synechococcus, and by larger eukaryotes and cryptophytes in the odra plume water. in the arabian sea during the ne monsoon, all ultraphytoplankton groups were subject to vigorous grazing by small protozoa (<10 microns). generally, around 100% (36 - 139%) of the cells produced were consumed per day, more or less sustaining a steady state system. at least two trophic steps within the nanoplankton (<20 microns) size range were detected. a removal of secondary predators (>10 microns) enhanced grazing on autotrophic ultraplankton in the <10 microns size range considerably. it is estimated that up to 83% of grazers <10 microns are removed by nano- and microzooplankton predators (10 - 200 microns) per day. during the ne monsoon, carbon consumption rates in the somali basin ranged between 4 - 28 microg dm-3 d-1, except for the gulf of aden and the southern red sea, where a much higher proportion of larger phytoplankton was consumed (71 - 146 microg dm-3 d-1). during the sw monsoon, phytoplankton carbon consumption rates were substantially higher at the eutrophic upwelling station (118 microg dm-3 d-1) than at the mesotrophic northern stations (48 - 86 microg dm-3 d-1) and at an oligotrophic southern station (20 microg dm-3 d-1). at the oligotrophic southern station, an experiment to estimate bacterivory showed bacterial consumption rates by microzooplankton to be very similar to phytoplankton consumption (25 microg dm-3 d-1). carbon consumption rates of phytoplankton in the pomeranian bay were in the same order of magnitude than in the eutrophic arabian sea stations (93 - 140 micro g dm-3 d-1). ultraphytoplankton carbon consumption in the gotland sea, analysed by flow cytometry, ranged from 58 - 119 microg dm-3 d-1.