<jats:title>ABSTRACT</jats:title> <jats:p> 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 <jats:italic>Roseobacter</jats:italic> 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 <jats:italic>Cytophaga-Flavobacterium</jats:italic> group (CF) of the <jats:italic>Bacteroidetes</jats:italic> . One <jats:italic>Cytophaga</jats:italic> sp. lineage with cultured representatives formed almost 20% of the CF group. Members of the <jats:italic>Roseobacter</jats:italic> cluster constituted approximately 50% of alpha-proteobacteria, but none of the <jats:italic>Roseobacter</jats:italic> -related isolates formed populations of &gt;1% in the environment. Thus, the readily culturable members of this clade are probably not representative of <jats:italic>Roseobacter</jats:italic> species that are common in the water column. In contrast, members of NOR5 were found at high abundances (&gt;10 <jats:sup>5</jats:sup> cells ml <jats:sup>−1</jats:sup> ) 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. </jats:p>