The coastline between Sao Sebastiao and Ubatuba cities and the shores of Sao Sebastiao, Anchieta and Mar Virado islands (Sao Paulo State, Brazil) are crosscut by several small swarms and isolated dykes trending N55E. The main rock types range from basic to intermediate, but also a conspicuous variety of alkaline lamprophyres occur side by side with the main group. The thickness of the basic to intermediate dykes vary widely, from a few centimeters to several metres, while the lamprophyres are a few tens of centimeters thick. The objective of this thesis is the petrographic, mineralogic, petrochemical and isotopic characterization of the basic and ultrabasic dykes that occur between the Sao Sebastiao and Ubatuba cities (State of Sao Paulo), also including some occurrences from the Sao Sebastiao, Mar Virado and Anchieta islands and from the Bairro Alto region (Folha de Natividade da Serra). The petrogenetic model presented is based in the national and international bibliography. (author) 146 refs., 44 figs., 24 tabs.