A wide range of aryloxyacetic acids and corresponding acids with alkyl groups in the side chain, their arylthio‐ analogues and the antibiotic griseofulvin have been assessed in the plate test for fungistatic effect on six fungi, and as systemic fungicides againstBotrytis fabaein broad beans andAlternaria solaniin tomatoes. The results indicate that in general, arylthio‐ derivatives are more fungicidal than their aryloxy‐ analogues. The systemic fungicidal performance of x‐(2‐chlorophenylthio)propionic acid in the tomato test at 1–100 p. p. m. was found to be of the same order as that shown by griseofulvin at 50–500 p. p. m. Variable results were obtained with griseofulvin in the tornato test and its performance in the bean test was consistently poor. Further evidence is presented which indicates that the protection conferred by certain compounds may not be due to a