Oocysts of Eimeria acervulina Tyzzer, 1929 were collected at the time of their first massive discharge into the chick intestine, and isolated in the cold by means of continuous centrifugal flotation in high‐density media. Maximum numbers (90%) of sporoblastic oocysts were obtained by incubating at 30°C for 10 hr and sporulation was considered complete within 20 hr. The initial Qo2 (dry‐weight basis) of 9.5 decreased linearly to < i/3 this value in 20 hr, and more slowly thereafter to very low levels. The QCO2 was higher than Qo2 only during the first 10 hr. A 15% decrease in dry weight in 15 hr was due chiefly to disappearance of alkali‐stable carbohydrate, probably identical with glycogen. In the later stages carbohydrate was resynthesized. Lipids were not oxidized during sporoblast formation, but disappeared rapidly betweem 10 and 30 hr. Total nitrogen remained constant, indirect evidence suggesting that protein was not used to supply oxidative energy The change‐over from carbohydrate to fat metabolism corresponded with observed changes in the respiratory quotient, and also with near‐completion of sporulation. Apparently carbohydrate provided the energy needed for the earlier stages of sporulation, whereas oxidation of lipids supported the teal stages as well as the metabolism during dormancy.