A series of thiosemicarbazones of saturated and α,β-unsaturated aliphatic aldehydes was prepared and tested in vitro for activity against various bacteria, yeasts, and molds. Antimicrobial potency was found to depend on chain length and was greatly enhanced by the presence of an unsaturated linkage. Thiosemicarbazones derived from unsaturated aldehydes with chain lengths C-10 to C-12 were the most active against the microorganisms tested.