According to the present invention, a microorganism belonging to the genus Aurantiochytrium having an 18S rRNA gene consisting of the base sequence represented by any of SEQ ID NOS: 1 to 5; Aurantiochytrium sp. OH4 strain; a microorganism which is a mutant obtained from the above-mentioned microorganism as a parent strain and has a higher ability to produce DHA than the parent strain; or Aurantiochytrium sp. LTR23 strain is provided. Also, a method for producing a DHA-containing composition, DHA and DHA alkyl ester by a fermentation process using the above-mentioned microorganisms is provided.