We tested new strategies for the isolation of abundant bacteria from coastal North Sea surface waters, which included reducing by several orders of magnitude the concentrations of inorganic N and P compounds in a synthetic seawater medium. Agar plates were resampled over 37 days, and slowly growing colonies were allowed to develop by repeatedly removing all newly formed colonies. A fivefold increase of colonies was observed on plates with reduced nutrient levels, and the phylogenetic composition of the culture collection changed over time, towards members of the Roseobacter lineage and other alpha-proteobacteria. Novel gamma-proteobacteria from a previously uncultured but cosmopolitan lineage (NOR5) formed colonies only after 12 days of plate incubation. A time series of German Bight surface waters (January to December 1998) was screened by fluorescence in situ hybridization (FISH) with isolate-specific and general probes. During spring and early summer, a prominent fraction of FISH-detectable bacteria (mean, 51%) were affiliated with the Cytophaga-Flavobacterium group (CF) of the Bacteroidetes. One Cytophaga sp. lineage with cultured representatives formed almost 20% of the CF group. Members of the Roseobacter cluster constituted approximately 50% of alpha-proteobacteria, but none of the Roseobacter-related isolates formed populations of >1% in the environment. Thus, the readily culturable members of this clade are probably not representative of Roseobacter species that are common in the water column. In contrast, members of NOR5 were found at high abundances (>10(5) cells ml(-1)) in the summer plankton. Some abundant pelagic bacteria are apparently able to form colonies on solid media, but appropriate isolation techniques for different species need to be developed.