The growth of Kloeckera apiculata (thiamine-less) was inhibited by deoxythiamine, but not the growth of Saccharomyces carlsbergensis which does not require thiamine for its growth. The inhibition of deoxythiamine for Kl. apiculata was recovered competitively by thiamine, and its inhibition index was 100-150. This compound had no thiamine activity. The order of decreasing antithiamine potency was : methylthiothiamine≒pyrithiamine> deoxythiamine> chlorothiamine> thiamine sulfuric acid ester> oxythiamine.