The production of bacterial magnetic particles (BMPs) by recombinant Magnetospirillum magneticum AMB-1 harboring the plasmid pKML was enhanced in pH I-regulated fed-batch culture. The addition of fresh nutrients was feedback-controlled as is function of the pH of the culture. The yield of BMPs was optimized by adjusting the rate of ferric iron addition, Feeding ferric quinate at 15.4 mug/min resulted in a BMP yield of 7.5 mg/l, which is the highest yield so far reported. Expression of a plasmid-encoded fusion protein and segregation of the plasmid during bacterial growth were both stable during fed-batch culture. More than 75 % of the cells retained the plasmid for 130 h under antibiotic-free conditions. In addition, the fusion protein permitting the display of a specific protein on the BMP surface was also stably expressed.