This study aimed at evaluating whether a magnetic field (MF) affects the growth of Spirulina sp. when applied to it at different exposure times in indoor and outdoor culture systems. The effects of MF on chlorophyll content, medium consumption and protein profile were also investigated. In raceway tanks, a 25 mT MF was applied for 24 h or for 1 h d(-1). MF for 24 h to outdoor assays increased biomass concentration and chlorophyll-a content besides altering the protein profile. Outdoor Spirulina growth was higher (similar to 3.65 g L-1) than the growth found in indoor assays (similar to 1.80 g L-1), while nitrogen and phosphorus consumption was not enhanced by the application of MF. This is the first study that investigated the influence of MF on outdoor microalga assays, and the results showed that MF affected the metabolism of Spirulina cultured in raceways, especially when it was grown outdoors in uncontrolled environmental conditions.