In an attempt to establish an archaeological chronology for northwestern Costa Rica, excavations were carried out in 1959-60 in the coastal region and Tempisque River drainage of Guanacaste Province. Four periods have been defined: Zoned Bichrome, Early Polychrome, Middle Polychrome, and Late Polychrome. These are roughly equated in the Maya sequence with Late Formative, Early Classic through the beginning of Late Classic, the latter part of the Late Classic through the early Postclassic, and late Postclassic, respectively. The Zoned Bichrome period has been established on the basis of three geographically separate but coeval phases: Chombo on the Santa Elena Peninsula, Monte Fresco in the Tamarindo Bay zone, and Catalina on the middle Tempisque. All three phases are linked to each other through trade pottery; a radiocarbon date on Monte Fresco is within the first century of the Christian era. Outstanding characteristics of the period are bichrome zoning, dentate rocker-stamping, wavy black lines produced by a multiple brush, engraving, and incising. Considerable fishing and hunting was carried out, and intensive maize agriculture is inferred. These village materials indicate that lower Central America was participating in at least some of the trait diffusion which linked remote areas of Nuclear America in Formative times.