The paper describes the first occurrence of Pleopis schmackeri (Poppe, 1889) (Branchiopoda, Onychopoda) in Rio das Ostras, Rio de Janeiro, Brazil is described. This marine cladoceran species occurred in zooplanktonic samples obtained on July, 2003 with mean density of 10 ind.m-3. Its presence may be related to two hypotheses, due to ballast water and through the Brazilian current, which gets closer to the coast Winter.
