Derivatives of 2,4-dioxo-5-benzylidene-3-thiazolidineacetamide were prepared and the fungicidal and the bactericidal activities of twenty-three related compounds were examined. In vitro tests at a concentration of 10 mg/l, some derivatives showed moderate or considerable inhibition against Pyricularia oryzae (P. oryzae), Rhizoctonia solani (R. solani), and Septoria tritici, but only modest activity against Botrytis cinerea and Burkhorderia gluma. The activity of a few compounds was enhanced by co-application of salicyl hydroxymate (SHAM), which is known to potentiate the fungicidal activity of strobilurins. In pot tests at a concentration of 500 mg/l, some compounds showed appreciable activity against rice blast, late blight on tomatoes, downy mildew on cucumbers, or leaf rust on wheat. Of these compounds N,N-dimethyl-2,4-dioxo-5-benzyl-3-thiazolidineacetamide completely removed the downy mildew. Most of them showed only modest activity against gray mold on cucumbers.