Four strains of budding, hyphal bacteria, which had very similar chemotaxonomic properties, were isolated from the Baltic Sea. The results of DNA-DNA hybridization experiments, indicated that three of the new isolates were closely related, while the fourth was only moderately related to the other three. Sequence signature and higher-order structural detail analyses of the 16s rRNA of strain IFAM 141gT (T = type strain) indicated that this isolate is related to the alpha subclass of the class Proteobacteriu. Although our isolates resemble members of the genera Hyphomicrobium and Hyphomonas in morphology, assignment to either of these genera was excluded on the basis of their markedly lower DNA guanine-plus-cytosine contents. We propose that these organisms should be placed in a new genus, Hirschiu baltica is the type species of this genus, and the type strain of H. bdtica is strain IFAM 1418 (= DSM 5838).Since the first description of a hyphal, budding bacterium, Hyphomicrobium vulgare (53), only the following additional genera having this morphological type have been formally described: Rhodomicrobium (9), Hyphomonas (40), Pedomicrobium (1, 14), Filomicrobium (49, and Dichotomicrobium (23). The taxonomy of these bacteria has been evaluated by using chemotaxonomic characteristics (56), DNA-DNA hybridization (15, 37), genome size determination (28), analysis of nucleotide distribution and DNA base ratio (16), and analysis of RNA cistron similarities (42). Phylogenetically , all of the above-mentioned genera are members of the alpha subclass of the class Proteobacteria, and they are not closely related to each other (51).In this paper we characterize genomically and phenetically four new isolates of budding, hyphal bacteria which have DNA base ratios of 45 to 47 mol%, values which are at least 10 mol% lower than the DNA base ratios of other hyphal, budding members of the Proteobacteria. MATERIALS AND METHODS Enrichment and isolation.At various times surface water samples (upper 5 cm) of the Kiel Fjord were taken from a boat landing. Different methods were used to enrich for budding, hyphal bacteria immediately after the samples arrived in our laboratory (Table 1). When a microscopic survey indicated the presence of such organisms, streaks were made on solidified M13 medium (44). Colonies were examined microscopically by using the toothpick procedure (23). Pure cultures were obtained within 6 months after sampling.Physiological tests. The isolates were grown in complex M13 medium, which contained 0.025% glucose, 0.025% peptone, and 0.025% yeast extract as carbon and nitrogen sources. Tests for utilization of carbon sources were performed in 100-ml Erlenmeyer flasks containing liquid M9 medium to which a filter-sterilized carbon source was added to a final concentration of 0.1% (wt/vol). Defined M9 medium contained 0.005% Na,HPO, and 0.1% KNO,. Metha-* Corresponding author. no1 and formamide were tested at concentrations of 0.02 and 0.1% (vol/vol). Utilization of nitrogen sources was tested in M9 medium containing gl...