ABSTRACT: In this work the diversity and distribution of magnetotactic bacteria (MTB) in Northern Germany were investigated. With the exception of extreme eutrophic environment different MTB (Spirilla, cocci, rods, vibrios) were found in all samples from marine and freshwater environments. Magnetotactic multicellular aggregates were found for the first time in the German Bight and in the Baltic Sea.During the incubation in microcosms an increasing number and a decreasing diversity of MTB was observed. In most cases the MTB population was dominated by magnetotactic cocci and 16S rDNA analysis identified them as Alphaproteobacteria . Up to 11% sequence divergences were found between the MTB of different microcosms but a high variability was also observed within single microcosms at different times.In one microcosm from a lake in Bremen a magnetotactic rod (MHB-1) was enriched which is closely related to Magnetobacterium bavaricum , the only known MTB from the Nitrospira phylum.A correlation between the development of a distinct MTB population in the microcosms and the original geographical location of the samples was not observed but the heterogeneous vertical distribution of MTB indicates an adaption to special gradients. Most MTB (up to 98%) were restricted to anoxic sediment layers and reached up to 1,5 x 107 MTB/cm3 with an abundance of up to 1% of the total cell counts which indicates a significant influence of MTB on the microbial iron cycle.The high effective enrichment of MTB by magnetic collection, race-track , resp. were demonstrated by microscopy, Denaturing Gradient Gel Electrophoresis (DGGE) of 16S rDNA fragments and Amplified Ribosomal DNA Restriction Analysis (ARDRA). The selective enrichment of MTB allowed the construction of a genomic library which most likely contains a fragment of the mamAB cluster.From the several cultivation experiments 10 new magnetotactic spirilla were isolated. All strains are microaerohpilic and members of the genus Magnetospirillum.