2-[2,4-dichloro-6-[(4-oxo-2-thioxo-5-thiazolidinylidene)methyl]phenoxy]-acetic Acid is used in biological activities in the preparation and aldose reductase inhibitory activity of a new series of 5-[[2-(ω-carboxyalkoxy)aryl]methylene]-4-oxo-2-thioxothiazolidine derivatives.