SEASONAL CHANGES IN A POPULATION OF DONAX TRUNCULUS IN EL BARQUERO ESTUARY (LUGO. NW SPAIN). - During the years 1985, 1986 y 1987 a sampling has been made of a sandy area in El Barquero estuary (Lugo. NW Spain) where there is a natural population of Donax trunculus. In the present work we have used the weighting data obtained during the three years in order to know the evolution of the total weight and the AFDW for each of the cohorts as well as for a standard animal of 27 mm size. Moreover, two condition indices have been applied and the percentage of mature individuals has been calculated for each sampling.