A microbicidal composition containing: (a) at least one non-halogenated 2-alkyl-4-isothiazolin-3-one selected from substituted and unsubstituted 2-(C.sub.1-C.sub.4)alkyl-4-isothiazolin-3-ones; and (b) at least one of 2,2'-dithiobis(N-methylbenzamide) and 2-methylbenzisothiazolone.