Relative to the high-sulfur diesel in use for decades, the U.S. Navy is facing exposure to new fuels that may behave differently in the marine environment. This laboratory work investigated the effects of biofouling and seawater exposure to high-sulfur diesel, ultralow-sulfur diesel, Fischer-Tropsch synthetic diesel, fatty acid methyl ester (FAME) biodiesel, and hydrotreated renewable diesel fuels. Bulk chemical changes were not detectable under the laboratory test conditions, but changes in fuel lubricity, controlled by traces of polar compounds in fuel, were detectable in several fuels by both physical and chemical testing. A marine microbial assemblage rapidly metabolized the lubricity improver (as C-14-linoleic acid) as it partitioned to seawater from hydrotreated renewable and ultralow-sulfur diesel fuels. Equipment dependent upon fuel lubricity, such as fuel pumps and fuel injectors, may be at risk when new fuels enter use in the near future, given that certain U.S. Navy ship classes store fuel wet, in seawater-compensated tanks. Our results have implications for improperly stored fuels, as well.