A 80-km long gravity traverse, with 43 stations, was made across the Lima segment of the Peruvian Coastal Batholith, eastwards from the coast at Huacho (lat. 11°S), through Sayán to Churin. A second traverse, 30 km long, with 14 stations, was made from near Sayán southwards to the coast at Rio Seco. Laboratory measurements of specific gravity were made on 74 samples, taken from all the major units of the batholith and its envelope. A Bouguer anomaly gradient of 5 mGal/km strikes parallel to the axis of the batholith. This gradient fits a model with an upper crust of 2650–2750 kg/m3 in density, which thickens from 2.5 km at the coast to 25 km at 125 km inland. This wedge of light rock includes the Coastal Batholith and the miogeosynclinal sediments. In addition, the model requires either a buried granitoid beneath Churin, at the eastern end of the traverse, or a lateral change of density in the lower crust. After the regional gradient is removed, the residual anomalies can be correlated well with the distributions and densities of acid and basic rocks in the batholith. Positive residuals over the acid central complex of the Rio Huaura are interpreted quantitatively in terms of bell-jar subsidence of gabbro-diorite blocks.