The effects of an external magnetic field on the production of porphyrin were investigated using Rhodobacter sphaeroides IFO12203 under anaerobic-light conditions. Upon application of a 0.13-0.3-T magnetic field, the growth was slightly suppressed and porphyrin extracellular production was activated at both the N and S poles, partiaelarly at the N pole up to about 5.3 times that in the control experiment (6.7 mg/l) (without magnetic filed application). The maximum production was 35.8 mg/l at the N pole with a magnetic field of 0.3 T. At the same time, the 5-aminole-vulinic acid dehydratase (ALAD) concentration was enhanced in the cells at the N and S poles, but particularly at the the N pole. This suggested the possibility that the magnetic field might activate ALAD gene expression in R. sphaeroides IFO12203 and result in enhanced porphyrin production.