The reproductive phenology and breeding success of a population of Montagu's Harrier (Circus pygargus) was studied in Southeastern Ciudad Real during 1988-1994. Montagu's Harriers arrive to the study area during the last week of March. Mean laying date was 28 April. Mean productivity was 1.4 fledglings/nest. The mean laying date showed significant differences between years. No between-year differences were found for clutch size, productivity and fledged brood size. Nestling mortality showed nearly-significant between-year differences. A significant decrease according to phenology was found for clutch size, hatched brood size, productivity and fledging success. Nestling mortality rate increased significantly throughout the breeding season. A comparison of these results with published data for other Iberian populations showed a significant delay of mean laying date with increasing latitude. No differences were found for clutch size, but mean productivity was higher in one population nesting in natural vegetation as compared to populations nesting in cereal crops.