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In Southeast Kalimantan of Borneo, the Martapura Cretaceous System is widely distributed. The stratigraphical and paleontological studies of the system were carried. out by mostly Dutch geologists during the end of the 19 th to the begining of this century. According to the workers, the Martapura System yields rich fossils such as pelecypods, gastropods, cephalopods and foraminiferas and the age of the system is Cenomanian to Senonian.In June of 1972 the present authors visited Southeast Kalimantan to establish the detail stratigraphy and collect useful fossils from the Cretaceous rocks along the upper stream of the Riam Kanan River, east of Bobalis mountainlands, where was already sank under the water after the completion of the Riam Kanan Dam in July of the year. As a result of the survey, they divided the Cretaceous strata called the Manunggul Formation into five units, Kalaan, Benuariam, Tabatan, Rantaulajon Formations, in ascending order. The Kalaan Formation unconformably overlies the schist and consists of conglomerates, sandstones and siltstones and contains Turonian ammonids, Reesidites sp., Inoceramus teraokai MATSUMOTO and NODA I. latus SOWERBY. The Benuariam Formation is composed of porphyritic lavas, agglomerates and tuffs, and conglomerates. The Tabatan Formation consists of sandstones and conglomerates. Orbitolina of Apto-Albian form is obtained from the limestone pebbles in the conglomerates of this and the Benuariam Formations. The Rantaulajon Formation is composed of fissil shale which contains rich estheriids, Pseudocyclograpta hashimotoi KOBAYASHI n. sp. and Lioestheria sp. These estheriids indicate that the formation is nonmarine origin and probable Senonian.The authors also survey Cretaceous strata distributed along the Riam Kiwa River and in Batotiris and its vicinity, west of Bobalis mountainlands. The Cretaceous strata exposed along the Riam Kiwa River were subdivided into four formations Buntutriam, Rantau Lingitan, Riam Lingitan and Atiin Formations, in ascending order. The Buntutriam Formation is composed of conglomerates of porhyrite cobbles and thin sandstone and mudstone layers. The Rantau Lingitan Formation comprises mudstone which yields late Turonian ammonoids, Subprionocyclus neptuni (GEINITZ). The Riam Lingitan Formation is made up of sandstone which contains many pelecypods. The Atiin Formation consists of purple colored tuffs and sandstones. The Rantau Lingitan and Riam Lingitan Formations are correlated with the Kalaan Formation and the Atiin Formation with the Benuariam Formation along the Riam Kanan River.Eocene to Miocene strata exposed along the Riam Kiwa River were also surveyed in detail. Based on the study of larger foraminiferas, the authors found that Td stage of Oligocene seems to be lacked in this area.