Forty-three putative species were found in this first-ever survey of the marine gastrotrich fauna of Brazil. Samples from 23 locations and 37 sites along the northern coasts of the state of So Paulo yielded 21 species of Macrodasyida (12 genera and five families) and 22 species of Chaetonotida (seven genera and two families). Based on morphology, 26 species appear to be taxa not previously described and thus far endemic to Brazil, 13 are known species, while one in each of the genera Diplodasys, Paradasys and Tetranchyroderma remains undetermined. The overall high diversity contrasts with the generally low number of species from single locations, averaging 4.3 (SD 3.5) spp. per location; very few species were found in sites characterized by very fine sand sediment loaded with detritus and by high water turbidity (e.g. 1 sp. in praia da Fazenda), whereas a well-diversified gastrotrich community was found in localities characterized by medium or fine sand with little detritus and clear water (e.g. 14 spp. in praia de Castelhanos). The macrodasyidans, Paraturbanella sp. 1, Pseudostomella sp. 1 and Tetranchyroderma sp. 1, and the chaetonotidan Heteroxenotrichula sp. 1 were the most common species, often sympatric and particularly frequent and abundant along the continental shoreline (recorded in 50-65% of these localities); on the other hand, Macrodasys fornerise n. sp., Ptychostomella sp. 1, Tetranchyroderma sp. 3 and Thaumastoderma sp. 2, while occurring sometimes in very high numbers, exhibited a much narrower range, being encountered only once or twice during the survey. The main morphometric traits are described of the 13 species considered to be cosmopolitan or with at least an amphi-Atlantic distribution, including Chaetonotus apechochaetus and Halichaetonotus marivagus, reported here for the first time from the Americas; of the 26 species thus far endemic to Brazil, Macrodasys fornerise n. sp., which is characterized by a noticeable 145 µm-long frontal organ, is also described and figured.