This paper presents the results of a series of investigations made on plankton samples (Table I) taken from October to December 1960 in the "Enseada do Mar Virado" (State of São Paulo, Brazil) (Fig. 1). The samples were studied under their general aspect. Four stations were occupied along a line at four different occasions. At each, samples were taken by means of vertical hauls with two similar plankton nets 25 cm wide at the mouth, one called "phytoplankton mesh" (1 mm wide apertures) and the other "zooplankton mesh" (3 mm wide apertures). First, a study on the quantitative composition of the plankton was carried out. A relationship was established between the amount of plankton and the oceanographic station (Fig. 2), date of hauls, water masses and other environmental conditions. The following results were obtained: the amount of plankton seems to be greater in Station I than in Station II, III and IV, which means that the amount of plankton was greater in coastal than in shelf waters. Phytoplankton on the contrary, was always more abundant in shelf waters (Figs. 3, 4). A comparative analysis was made of the samples taken with phytoplankton and zooplankton meshes. It was verified that the phytoplankton mesh really selects only small animals and that larger organisms are not retained by any of them (Figs. 5-12). Finally a study on the relationship between the planktonic animals (Table II) and the habitat, especially in what concerns the zooplankton, revealed that the latter is essencially neritic.