The smooth clam Callista chione is an Atlantic-Mediterranean species that is commercially exploited in several European countries. Several aspects of its reproduction were studied in a coastal location of the northwestern Mediterranean as a tool for sustainable fisheries management. Gonadal development was classified into 6 different stages, ranging from immature to degradation of the remaining sexual structures. Results showed that C. chione was able to reproduce throughout the year but a main spawning period occurred in spring. First records of hermaphrodites were found for this species. The mature spermatozoa measured 4.70±0.66 μm (without the flagellum), including a very short acrosome of 0.75±0.09 μm. The mid-piece of the spermatozoa had a mitochondrial sheath composed of five mitochondria. Ripe oocytes ranged from 60 to 80 μm in length. Size at first maturity (SL50) was estimated for the first time in a Mediterranean population, showing values of 30.26 mm in 2005-2006 but only 21.45 mm in 2014.