The present paper deals with the morphology and population structure of Diopatra neapolitana collected in Izmir Bay between July 1997 and June 1998. The density of the species in the area ranged from 42 ind(.)m(-2) (November 1997) to 198 ind(.)m(-2) (March 1998), the biomass from 84 g(.)m(-2) (November) to 408 g(.)m(-2) (March). The coelom contained gametes in all months, except for January, indicating that it has a very long reproductive period in the area. The relationships among the biometrical measurements taken on specimens of Diopatra neapolitana from Izmir Bay were assessed by using regression and correlation analysis. A re-description of the species is provided.