Requirements of sows may change in pregnancy because of maternal tissue development and conceptus growth during the different phases of gestation. Isoleucine (Ile) is a limiting amino acid (AA) in corn-soy diets. The objective of this study was to determine the Isoleucine requirement in early (EG) and late (LG) gestation using the indicator AA oxidation method.   It was found that Ile requirements of adult sows increased more from EG to LG than tryptophan requirements in young, growing sows, and were driven mainly by maintenance in EG and by fetal growth in LG. The AA supply is more critical in LG than energy intake for adult sows. Therefore, phase feeding during gestation is necessary to meet the sows’ requirements.