The magnetotactic bacterium was isolated from freshwater sediment from North Lake of Wuhan. The isolate, designated WM-1, was Gram-negative, helical shaped, and studied by means of electron microscopy. The strain WM-1 was 0.2-0.4 microm in diameter and 3-4 microm in length. The DNA G + C content was found to be 65.7 mol%. Phylogenetic analysis of the 16S rDNA gene (Accession number DQ899734 in GeneBank) revealed that this isolate was a member ofalphasubdivision of the Proteobacteria. Strain WM-1 was closely related (97.7%) to Magnetospirillum sp. AMB-1. Randomly amplified polymorphic DNA analysis showed that these two strains were in fact different strains. Electron diffraction patterns of WM-1 magnetosomes indicated that the magnetosomes were composed of magnetite. The magnetosomes from WM-1 were cuboidal in shape as observed by electron microscopy. Statistical analysis of magnetite crystals from WM-1 showed narrow asymmetric size distribution. The average number of magnetosomes in each WM-1 bacterium was 8 +/- 3.4. The average length of magnetosomes in WM-1 was 54 +/- 12.3 nm and the average width is 43 +/- 10.9 nm. These data showed that the grains in WM-1 were single-domain crystals.