A dense sampling of planktonic foraminifers is described from the Cercado, Gurabo, and Mao Formations in the Cibao Valley, northern Dominican Republic. Assemblages are relatively consistent throughout, with Globigerinoides spp. dominating each section. The Cercado Formation is loosely confined to Zones N17 and N18 (∼8.6-5.6 Ma). The Gurabo Formation spans Zones N18 and N19 (∼5.6-4.5 Ma). The Mao Fm. is placed in Zone N19 (∼4.5-3.6 Ma). The predominant genera (Globigerinoides and Orbulina) and species indicate a near-shore setting throughout the period of deposition with an episode of deepening after a lowstand at the Miocene/Pliocene boundary. Increasing abundances of Gs. sacculifer and Gs. ruber in conjunction with a decreasing abundance of G. bulloides suggests the onset of increasing salinity and diminishing primary productivity at ∼5.8 Ma. A sharp increase in Gs. sacculifer and Gs. ruber at ∼4.8 Ma implies a major intensification of Caribbean surface water salinity with a decline in primary productivity. The mechanism for this change is isolation of the Caribbean Ocean through enhanced closure of the Central American Seaway (CAS). Upwelling events are recorded in the upper Cercado Fm. (∼6.1 Ma) and in the middle Mao Fm. (∼4.2 Ma) by spikes in G. bulloides and Neogloboquadrina spp. respectively. The timing of increasing salinity and decreasing productivity (∼4.8 Ma) and upwelling (∼6.1and 4.2 Ma) generally corroborate previously suggested paleoceanographic changes related to the uplift of Panama. Oceanographic changes depicted by paleobiogeographic distributions suggest that shoaling along the Isthmus of Panama had implications in the Caribbean as early as 6.5 Ma. Major paleobiologic changes between ∼4.8 and 4.2 Ma likely represent the period of final closure of the CAS and the complete division between Pacific and Caribbean water masses.