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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.