Causes of clutch loss and nestling mortality have been studied in 134 nests of Montagu's Harrier in the SE of Ciudad Real (central Spain) during 1988-1994. In this area, Montagu's Harriers nest in cultivated lands (mainly cereal crops). Clutch loss affected up to 18% of nests. Nestling mortality affected up to 47% of chicks. The main cause of breeding failure in this population was cereal harvesting. Harvest mortality has been estimated about 70% of nestlings in this area when no protection activities are conducted. Relationships between mortality, type of crop used for nesting and date of breeding are discussed.