A microbicidal composition contains as active ingredient 2-chloro-4-trifluoromethylthiazol-5-carboxylic acid derivatives of the formula I ##STR1## R is an organic radical with up to 40 carbon atoms, which optionally contains nitrogen, oxygen or sulfur atoms and which can be transformed by hydrolosis or oxydation into the directly to the thiazol ring bound carboxyl rest.These compounds have good microbicidal activity and are used for the control and prevention of infestation of plants by phytophathogenic microorganismes.