Seasonal variation in species composition, biomass, and growth rate of ciliates was investigated in the southern Kattegat, Denmark. The biomass was dominated by the autotrophic Mesodinium rubrum and naked mixo- and heterotrophic oligotrich ciliates, while tintinnids were less abundant. The seasonal distribution of both M. rubrum and the total biomass of all other ciliates largely followed variation in phytoplankton abundance, with a distinct spring peak, a less pronounced early autumn peak, and lower biomass in summer and late autumn. Temperature explained 75–97% of the variation in specific growth rates in the four size groups of mixo-heterotrophic ciliates for which the most complete seasonal data were available. The average Q₁₀ was 2.6 (range of 2.2–3.0). Growth rates were independent of various measures of food availability. Average temperature-corrected specific growth rates of all but the smallest (10–15 µm) bacterivorous ciliates were allometrically related to body volume (r² = 0.92, exponent = −0.24) and close to maximal growth rates obtained in laboratory cultures, which suggests that the ciliate community was rarely or never limited by food availability. The calculated clearance potential of the copepod community was sufficient to check the community biomass of ciliates ≥50 µm throughout the year.