Objective To clarify the effect of Fe3+ concentration on magnetite formation, Magnetospirillum magnetotacticum MS-1 was cultured at various initial concentrations of Fe3+ and the yield and magnetic properties of the cells were investigated. Results Although total cell yields (g/l) were not dependent on the initial concentration of Fe3+ (0-68 mu M) in the growth medium, the percentage of magnetic cells increased as the initial Fe3+ concentration increased. The coercivity (H-c) and saturation magnetization (M-s) of dried MS-1 cells increased and the blocking temperature (T-B) decreased as the initial Fe3+ concentration increased. These values are similar to those reported previously. The Verwey transition temperature (T-V) was almost identical for all initial Fe3+ concentrations (95-97 K), except for medium without added Fe3+ (0 mu M; 88 K). Conclusion Fe3+ concentration strongly affects the magnetic properties of MS-1 cells, especially in the lower concentration range, suggesting that the Fe3+ concentration should be considered when evaluating the magnetic properties of MS-1 cells.