We report the first occurrence of Oithona davisae Ferrari F.D.&Orsi, 1984 in the Sinop coast, Turkey. Data on the abundance and biomass of this species, and the proportion of adult and copepodite stages in the total copepods were investigated. The zooplankton samples were collected monthly by vertical hauls with plankton net of 112 mu m mesh size from January to December 2009. However, O. davisae was observed for the first time in September, and continued to be observed for the remainder of the sampling period. The highest abundance and biomass of this species were recorded in November 2009 (1485.35 ind.m(-3) and 2.9 mg.m(-3), respectively), which was 39.89% in the abundance and 14.35% of the biomass of the total copepods in November. In conclusion, the non-indigenous copepod species, O. davisae, has adapted to the ecological conditions of the Sinop coast.