The concentrations of heavy metals in the bivalve molluscs Anadara similis and A. tuberculosa are determined. The specimens were collected at 2 km from the mouth of Estero Huaylá, El Oro Province, Ecuador, in November 2013. The length and weight of each species were determined in the laboratory, with two length ranges being established for A. similis and four for A. tuberculosa. The meat of the organisms was extracted, washed and dried in an oven at 60 ° C for 72 h, then digested with a mixture of HNO3 and HClO4 in a 3: 1 ratio. The concentrations of 4 metals: lead, arsenic, mercury and cadmium were determined by atomic absorption spectrophotometry. The mean values obtained in A. similis (mg / kg) were: Pb (8.51 ± 0.34); As (1.42 ± 0.06); Hg (618.7 ± 355.32); Cd (1.21 ± 0.23) mg / kg; While for A. tuberculosa: Pb (7.52 ± 0.46); As (1.55 ± 0.14); Hg (364.38 ± 91.39); Cd (1.68 ± 0.28) mg / kg. The recorded concentrations of Pb, As, Cd and Hg in both species of molluscs exceed the maximum limits allowed by the International Standards. The smaller sizes (3-4 cm) of A. tuberculosa have the capacity to bioaccumulate more Pb, Hg and Cd, while A. similis showed its highest concentration of Pb in its smallest size (4-5 cm).