The destructive activity of mildew on cotton cloth makes desirable a search for new organic compounds which show mildew- proofing activity. Rhodamine, which contains the dithiocarbamate structure as a part of a cyclic structure, did not protect cloth in pure culture or soil burial tests. Situ derivatives of rhodamine, formed by condensation with aldehydes or ketones, containing an unsaturated carbonyl group as well as the dithiocarbamate structure, have been screened as protectants for cotton cloth. Eight compounds have been found which in 2% concentration on cotton cloth are sufficiently active to prevent the loss of more than 30% of the tensile strength after 4 weeks' burial in composted greenhouse soul. The size and overall shape of the rhodamine derivative and, with arylidene derivatives, the nature of the substitutent attached to the phenyl group, are important factors in the mildew-proofing activity of compounds of this series.