This paper deals with the Diatomacea from the plankton collected at the coast of the States of São Paulo and Rio Grande do Sul (Brazil), as well as with some specimens gathered along the brazilian coast (up to S.Pedro and S.Paulo Rocks), by Eng. J.Weiher, during his voyage to Europe. Three collections were examined from which 182 Diatomacea were determined as occurring in the Brazilian coast, as many as 126 of tnem being also peculiar to Uruguay coast while 114 were recorded from Argentine w aters. The material from Brazil was compared with that from the coasts of Uruguay and Argentine. The author points out that not all of them are planktonic speoiea since the material collected contained neritic as well as fresh water specimens. Among them there were strayed individuals that reached the marine plankton- Neritic species collected nearer the coast were also present in the material collected by Eng. Weiher, specially when the vessel approached Santos and Rio de Janeiro. This fact confirms the displacement of neritic material towards the open sea. For instance, Hemidiscus ova lis was also found in the open sea, in off shore samples from S.Francisco do Sul. Hemidiscus membranaceus and Asterolampra marylandica, both typically tropical species were found from Rio de Janeiro northwards. Chaetoceros coarctatus, frequent species in the Erazilian samples may be found in Uruguay during the winter months, when it appears all of a sudden and soon disappears. According to the author, it might well be an individual carried by the Erazilian Equatorial Current. These are, therefore, doubts that occur to all who devote themselves to the study of plankton and further investigations are necessary in order to find the answer to these questions. As a conclusion the author reminds that this paper was written with the purpose of serving as a guide for future investigations and it is far from being complete. Attention is called to the fact that it is necessary to carry on with systematic studies with the plankton from the Brazilian coast, including not only the Diatomacea but also all the micro-organisms that live in the marine environment.