Bacterioplankton growth in temperate Lake Zurich (Switzerland) was studied during the spring phytoplankton bloom by in situ techniques and short-term dilution bioassays. A peak of chlorophyll a (Chl a) concentrations was followed by a rise of bacterial cell numbers and leucine assimilation rates, of the proportions of cells incorporating 5-bromo-2-deoxyuridine (BrdU), and of community net growth rates in dilution cultures. Incorporation of BrdU was low in Betaproteobacteria (2% ±1), and pronounced growth of these bacteria in the enrichments was only observed after the decline of the phytoplankton bloom. An initial peak in the proportions of BrdU positive Actinobacteria (30%) preceded a distinct rise of their cell numbers during the period of the Chl a maximum. Cytophaga-Flavobacteria (CF) changed little in numbers, but featured high proportions of BrdU positive cells (28% ±12). Moreover, CF represented >90% of all newly formed cells in dilution cultures before and during the phytoplankton bloom. One phylogenetic lineage of cultivable Flavobacteria (FLAV2) represented a small (0.5-1%) but highly active population in lake plankton. The growth rates of FLAV2 in dilution cultures doubled during the period of the Chl a maximum, indicating stimulation by phytoplankton exudates. The high in situ growth potential of these bacteria was probably counterbalanced by top down control. Thus, CF, and specifically Flavobacteria, appeared to be substantially more important for carbon transfer in Lake Zurich spring bacterioplankton than was suggested by their standing stocks.