Bivalves of the genus Donax are commonly dominating the macrofauna of exposed sandy beaches (Ansell 1983). The family is worldwide distributed and comprises 64 species. At Playa Jahuay (S13°2433, W76°1149) located 180km south of Lima a dense population of D. marincovichi is present since decades. During a one year study monthly samples are taken to analyse the population dynamic. First results indicate distinct cohorts. Histology carried out will be carried out to analyse the reproduction cycle of this species showed separate sexes and no hermaphrodites.