We investigated the influence of a magnetic held on photosynthesis in, and the growth of Spirulina platensis (S. platensis), under magnetic fields with magnetic flux densities varying from 0.5 gauss (geomagnetic field) to 700 gauss. The specific growth rate of S. platensis was the highest at 100 gauss, being 1.5-fold that at 0.5 gauss, while the growth was obviously inhibited at 700 gauss. The existence of a magnetic field had no effect on the growth when S. platensis was cultured in the dark on a medium containing 0.3% glucose. The oxygen evolution rate during photosynthesis increased with increase in magnetic flux density, and the intracellular concentrations of sugar and phycocyanin, a light-harvesting pigment present in the thylakoid membrane in which reactions of the photosynthetic electron transfer system occur, reached maximal values at 100 gauss. At magnetic flux densities of a over 100 gauss, the phycocyanin content decreased with increase in magnetic flux density. The content of glyceroglycolipid, which exists exclusively in the thylakoid membrane, decreased with increase in magnetic flux density, especially so at 700 gauss, when it became 44% that at 0.5 gauss. From the aforementioned results, it is evident that magnetic fields accelerate the growth of S. platensis associated with activation of light excitation in the photosynthetic electron transfer system and increase in phycocyanin contents, and that these effects are maximal at magnetic flux densities of around 100 gauss. Nevertheless, when a magnetic flux density of over 400 gauss was applied, growth inhibition was observed with decrease in the phycocyanin content, and production of chemical energy necessary for sugar synthesis.