We investigated the relationship between the synthesis of bacterial magnetic particles (BMPs) and the transcription of magA gene-encoding iron transport protein using synchronous culture of Magnetospirillum magneticum AMB-1. Synchronously cultured cells were subjected to transmission electron microscopic observation and fluorescence in situ hybridization, The average number of BMPs slowly increased in the cell with increasing cell size. A sharp increase in BMPs occurred just before cell division and resulted in maximum BMP production of 30 particles/cell. The transcription of magA tvas regulated immediately before and after cell division.