of thermophilic F 1 -ATPase were prepared, and their catalytic properties were compared to know the role of ␦ and ⑀ subunits in catalysis. The presence of ␦ subunit in the complexes had slight inhibitory effect on the ATPase activity. The effect of ⑀ subunit was more profound. The (؊⑀) complexes, ␣ 3  3 ␥ and ␣ 3  3 ␥␦, initiated ATP hydrolysis without a lag. In contrast, the (؉⑀) complexes, ␣ 3  3 ␥⑀ and ␣ 3  3 ␥␦⑀, started hydrolysis of ATP (<700 M) with a lag phase that was gradually activated during catalytic turnover. As ATP concentration increased, the lag phase of the (؉⑀) complexes became shorter, and it was not observed above 1 mM ATP. Analysis of binding and hydrolysis of the ATP analog, 2 ,3 -O-(2,4,6-trinitrophenyl)-ATP, suggested that the (؉⑀) complexes bound substrate only slowly. Differing from Escherichia coli F 1 -ATPase, the activation of the (؉⑀) complexes from the lag phase was not due to dissociation of ⑀ subunit since the re-isolated activated complex retained ⑀ subunit. This indicates that there are two alternative forms of the (؉⑀) complex, inhibited form and activated form, and the inhibited one is converted to the activated one during catalytic turnover.