The present paper is the first of a series dealing with marine zoogeography of Brazil. The Brazilian marine fauna has been well worked only recently, and the contributions on the zoogeography of various groups are in dependence of conclusions of faunistic surveys. Since Dana (1852) to present, various authors have divided diversely the littoral in natural regions, according temperature, salinity and species ranges. In the present paper the littoral between French Guiana and North Argentina was divided in 26 strips 2º wide, each strip extending from the coast to the 200 m isobath. The temperature and salinity data (surface and bottom) collected in each strip by Oceanographie Ship "Almirante Saldanha" were gathered in order to calculate the mean temperature of extreme months and the corresponding salinities (other data were also used, when necessary). The species list of each strip was made with the previously published records and the data of research institution files. The distributional data for the hermit crab were analysed in three ways: horizontal distribution; bathymetric range and substrate preferences. It was possible to divide the study area in five provinces (Guyanian, Brazilian, St. Paul's, Uruguayan and Argentinian); the St. Paul's Province is a transition zone between Tropical and Anti-Boreal Regions.