Requirements of sows may change in pregnancy because of maternal tissue development and conceptus growth during the different phases of gestation. The objective of this study was to determine the Ile requirement in early (EG, 37 to 61 d) and late (LG, 89 to 109 d) gestation using the indicator amino acid oxidation method.  It was found that energy retention was similar in EG and LG, but the respiratory quotient decreased from EG to LG  and decreased with increasing dietary Ile level, indicating lipid mobilization in LG when Ile was at or above the requirement. The increase in Ile requirement from EG to LG suggests that phase feeding during gestation is necessary. Diets for LG should contain more Ile and be fed at greater allowances than in EG to meet the sows’ demands for nutrients.