PROBLEM TO BE SOLVED: To provide useful products such as fatty acids utilizing microorganisms of the thraustochytrid species and usages thereof.SOLUTION: The present invention provides newly isolated strains of the thraustochytrid species, and mutants obtained from the strains by irradiation treatment or the like being rich in docosahexaenoic acid and the like. Based on the strains, the invention also provides cultures, biomasses, extracted microbial oils, and their compositions. Further provided are methods of applying the biomasses, the microbial oils and the compositions to food, feed, cosmetics and pharmaceuticals.