Fish mycoses and single-cell ectoparasites on fish are controlled using an active compound suitable for inhibiting the mitochondrial respiratory chain at the stage of the b/c.sub.1 complex.

Use of strobilurins and oudemansins as biocides in aqueous systems is disclosed. Uses include wet-lap preservation, mold proofing, mildew proofing, and other uses where anti-fungal and/or anti-mildew activity is desired. Aqueous systems treatable with strobilurins and/or oudemansins include pulp and paper systems, oil and gas systems, cooling towers, and heat exchangers.