In the summer of 1996, a detailed survey of natural phytoplankton samples from the freshwater part of the Scheide estuary was carried out to check for the occurrence of poorly studied heterotrophic protists, the herbivorous amoeba Asterocaelum algophilum and chytrid parasites. The plankton community during the summer bloom was dominated by centric diatoms (Cyclotella scaldensis, C. atomus). Fungal infestations of these bloom-forming species were not observed in fixed samples, although severe infections were frequently observed in net samples which were maintained in the laboratory for a few days. The chytrid infecting the diatom Actinocyclus normanii was identified as Podochytrium cornutum. Four morphologically distinct chytrids resembling the genus Podochytrium parasitized cells of C. scaldensis. In contrast to the populations of centric diatoms, infection of the riverine Scenedesmus population by the chytrid Rhizophydium scenedesmi occurred in situ. The infection prevalence was maximum ca. 24%, leading to a twofold reduction in the biomass of Scenedesmus. It is hypothesized that the condition of the host population is a discriminating factor since only the allochtonous phytoplankton populations get infected, when entering the estuary. On the other hand, Asterocaelum algophilum was shown to feed voraciously on centric diatom cells. Cyclotella atomus, C. scaldensis and Stephanodiscus hantzschii were its main prey; infrequently, some green algae (Crucigenia) were found ingested too. At times, the amoeba was capable of grazing away ca. 25% of the diatoms per day. On average, larger diatom cells were found relatively more frequently inside 'feeding cysts' of this unusual amoeba when compared to the undigested centric diatom populations, which suggests the occurrence of some size-selectivity in the feeding behaviour of A. algophilum. Since some-times more than 70% of the C. scaldensis cells and more than 50% of the S. hantzschii cells were found ingested, the impact of grazing by A. algophilum on the populations of the larger diatom species in the estuary can be significant. As a result, more attention should be paid to the presence and role of these organisms in aquatic food web studies.