Spatio-temporal distribution and population structure of Enoplus quadridentatus Berlin, 1853 associated with mussel beds were investigated seasonally (October 2013, January 2014, May 2014 and July 2014) along the coasts of Sinop Peninsula, the northernmost point of Turkey located at the Black Sea. A 20x20 cm quadrat was used by scuba divers to collect samples at five stations, of which four of them were located on rocky substrata and one on a concrete pier pile. Nematode densities ranged between 0 – 996 ind.m-2 and found high in October and May. A total of 1100 individuals and 9 species belonging to 8 families were recruited. E. quadridentatus Berlin, 1853 was the dominant species in most of the nematode assemblages inhabiting mussel beds and had a contribution of 74% to the total fauna. The density of the species varied among the seasons with peaks in May and October, consistent with the general nematode densities. Population composition revealed seasonal changes. In general, females were dominant in the population and ovigerous females were recorded only in January and May. Juveniles were found during all seasons, but formed dense populations in July and May. It seems that the reproduction of the species is continuous throughout the year.