IT has been reported recently1 that Staphylococcus aureus, when cultured successively in media (glucose, 3.0 g; hydrolysed vitamin-free casein, 4.0 g; di-potassium hydrogen phosphate, 5.0 g; cysteine hydrochloride, 0.02 g; nicotinic acid, 0.02 g; thiamine hydrochloride, 10 μg; dist. water, 1 l.; pH 7.0) containing increasingly higher doses of pyrithiamine, acquires resistance to this antivitamin. After this adaptation, the bacteria can grow well in the presence of pyrithiamine (1 μg/ml.) even in the absence of thiamine with some changes in the metabolic pattern. Of the major biochemical alterations as a result of adaptation, severe depression in glucose utilization with the concomitant stimulation in the utilization of acetate has been noticed. By measuring the rate of formation of carbon dioxide labelled with carbon-14 from glucose-1-14C and glucose-6-14C, it has been observed that the percentage reduction in 14CO2, formation from glucose-1-14C by the adapted organism is much more than that obtained from glucose-6-14C, causing thereby a lowering in the glucose-1-14C/glucose-6-14C metabolic ratio after adaptation. Relative to the normal strain the adapted strain utilizes acetate-14C at a much faster rate both in the formation of 14CO2 and also in the incorporation of carbon-14 into the protein and lipid fractions of the bacterial cells. It has further been noted that as a result of adaptation to pyrithiamine the colour of the pigment of the organism is changed from orange-yellow to lemon-yellow. © 1962 Nature Publishing Group.