The present invention provides a microorganism which, when an underwater structure is formed with a coating film containing it, produces a biojelly in water to prevent fouling of the structure with macroscopic aquatic life, such a coating or a coating film, and a method of preventing attachment of macroscopic aquatic life which comprises using said coating or coating film. The present invention is directed to a strain of microorganism belonging to the genus Alteromonas and having the ability to produce a biojelly.