Mutagenicity and co-mutagenicity of strong static magnetic field up to 5 T was estimated in budding yeast Saccharomyces cerevisiae XD83. We observed that exposure to a 5 T static magnetic field resulted in a slight but significant increase in gene recombination frequency while reverse mutation was not altered. This mutagenic effect showed a dose response relationship as J-shape. In co-mutagenicity assay, it was found that exposure to a 5 T static magnetic field has tendency to enhance the mutagenic potential of Zeocin (Phleomycin) that is an antibiotic agent by free radical production. These results suggest that strong static magnetic field would have small but detectable mutagenic and possible weak co-mutagenic potential. However, the extent is estimated extremely small by comparison with other mutagens such as ultraviolet irradiation.