A series of α-isocyanoacrylic acid derivatives were prepared via the α-formylaminoacrylic acid derivatives, which were obtained by the reaction of isocyanoacetic acids and carbonyl compounds, in order to examine their antimicrobial activities. The structureactivity relationships are presented ; it was found that (Z)-α-isocyanoacrylic acid esters possessing halogeno phenyl or thienyl groups exhibited fairly strong antifungal activities. Of these, (Z)-methyl α-isocyano-β-(2-thienyl) acrylates (5s and 5u) showed the highest activity (1.56μg/ml MIC) against Tricophyton mentagrophytes.