Recently exposed Oligocene-Miocene (O-M) strata in Santa Elena, Guyanilla and the section at Playa de Jaboncilla, southwestern Puerto Rico show variations in lithology and in the occurence and relative abundance of planktonic and benthic foraminifers. The planktonic foraminifers are interspersed in the relatively deeper, open marine shelf segments, wheras the intervening shallower segments are characterized by an abundance of benthic foraminifers. Although shallow marine, smaller benthic foraminifers are generally of limited use for biostratigraphy, they are used in conjunction with planktonic taxa to develop a high-resolution biostratigraphy for the Oligocene-Miocene reefel carbonates of southwestern Puerto Rico. Upper Oligocene mudstone and packstone and overlying Miocene argillaceous limestone and chalk yielded a diverse foraminiferal fauna comprising both planktonic (Catapsydrax, Chiloguembelina, Dentoglobigerina, Globigerina, Globigerinoides) and benthic (Anomalinoides, Arcaias, Cancris, Elphidum, Eorupertia, Hanzawaia, Heterostegina, Lepidocyclina, Miogypsina, Miosorites, Neorotalia, Rosalina, Siphonina, Stilostomella and Yaucorotalia) taxa. This assemblage indicates a late Oligocene-Miocene (P21 - M14 Zone)age for the Santa Elena and Playa de Jaboncilla sections, with a break in sedimentation at the top of the upper Oligocene. Significant shallowing occurred between the upper Oligocene and Miocene resulting in a hiatus, comparing well with the global seal-level regression at that time. This hiatus lasted for 3.3 million years as shown by the absence of planktonic foram Zone P22 (27.1 Ma - 23.8 Ma).