shell length frequency distribution (slfd) and sex ratio of a subpopulation of the mangrove cockles anadara tuberculosa and anadara similis were analyzed from natural beds in the cayapas-mataje ecological mangrove reserve, ecuador. the slfd was different between sexes for both species; females were larger than males. subpopulations of a. tuberculosa and a. similis were dominated by females throughout the year. the overall sex ratio was 2.6:1 (female: male), although with variations in the monthly samples, from 1.2:1 to 5.9:1 in a. tuberculosa and 1.9:1 to 5.3:1 in a. similis. possibly, these results may be due to differential growth rate between sexes and environmental factors along the year.