Temperature and available food are the main factors in the variation of the growth rate in Mollusca. In the intertidal zone, both factors undergo important fluctuations with the tidal rhythm of exposure to the air. Their intensity and duration, together with other factors, are the cause of growth inhibition. In this article growth rates of a population of Cerastoderma edule L. which is permanently located in the subtidal zone of the Palmones River estuary in Algeciras Bay (Southwest Spain) have been studied. Cohort identification was made by the method of analysis of size frequency distribution (HARDING 1949; CASSIE, 1954) using probability paper. Five cohorts per year developing according to VON BERTALANFFY’S (1938) model were found, being recruited from April to September. Causes of growth rings formation have also been analysed; some evidence has been obtained about the relation between size and the probability to have growth rings; their respective probability functions are described. Results show, as expected, growth rates higher than those obtained by other authors (FIGUERAS, 1956; CAMACHO, 1984; MEJUTO, 1984; IGLESIAS, 1986) studying intertidal populations of C. edule in neighbouring temperate areas. This is due to the continuous provision of food and low range of temperature fluctuations in the subtidal, as opposed to the periodic starvation and important changes of temperature, because of tidal cycles, that the intertidal populations suffer.