A new species of single-cell magnetotactic bacteria, the NMV-1 bacteria, has been found by us. Each NMV-1 bacterium synthesizes itself a chain of magnetic nanoparticles inside its body. When an external magnetic field is applied, long bacteria chains come into being in the direction of the field in the water. More than 30 µm long chains of live bacteria have been observed. Length of bacteria chains is field dependant: the higher the magnetic field is, the longer the bacteria chains are. The bacteria chains orientation is controllable and the chains can be stably trapped. The mechanism of the assembly of long bacteria chains is also discussed. The results show that, while an external magnetic field is applied, the NMV-1 bacteria have strong enough interactions between each other to assemble long bacteria chains. After positioning the bacteria chain, cellular membranes of the bacteria were removed by cell lysis, leaving long chains of magnetic nanoparticles on a substrate. These magnetic nanochains can be potentially used as building blocks for magnetic nanostructures.