Seasonal changes in the species composition, abundance and biomass of planktonic ciliates were determined every 2-3 weeks at two sites of 30 m depth and one location of I05 m depth in the south-A total of 40 ciliate taxa were observed during this period. Autotrophic Mesodinium rubrum dominated ciliate abundance and biomass; maximal values of 5 0 . 10' ind. I-' and 65 pg C I-' were recorded. The annual mean biomass of M . ruhrum comprised 6 to 9% of the annual mean phytoplankton biomass. The highest abundances and biomasses of heterotrophic ciliates were noted at all stations in the spring and summer in the euphotic zone with maximum values of 2 8 . lo3 ind. I-' and 23 pg C I-'. Three ciliates assemblages were distinguished in the epipelagic layer: large and medium-size non-predatory ciliates, achieving peak abundance in spring and autumn; small-size microphagous ciliates and epibiotic ciliates which were abundant in summer, and large-size predacious ciliates dominating in spring. Below 60 m, a separate deep-water ciliate community composed of Prorodon-like ciliates and Meincystis spp. was found. The ciliate biomass in the 60-105 m layer was similar to the ciliate biomass in the euphotic zone. The heterotrophic ciliate community contributed 10 to 13% to the annual mean zooplankton biomass. The potential annual production of M . ruhnrm comprised 6 to 9% of the total primary production. Carbon demand of nonpredatory ciliates, calculated on the basis of their potential production. was estimated to be equivalent to 12-15% of the gross primary production., , , = maximum growth rate (d-'), T = temperature ("C), V = mean cell volume in an individual size-class (pm'). The total production for each sample was calculated by summing individual production values for different size-classes. Results I . Taxonomic Composition and Abundance of CiliatesAlthough silver impregnation techniques for ciliate identification have been established (MONTAGNES and LYNN, 1987), these procedures are rather time consuming and difficult to apply in ecological field studies (material for this study was collected before the QPS