We compared the ultrastructure of spermatozoa from the bivalves Anomalocardia brasiliana and Tivela mactroides (Veneridae). The spermatozoa of both species were of the ectaquasperm type in which the head contains a curved nucleus with a short cone-shaped acrosome. An invagination penetrated almost the entire length of the acrosome. The midpiece contained a pair of orthogonally arranged centrioles surrounded by spherical mitochondria and the flagellum had the typical 9 + 2 structure The spermatozoa of A. brasiliana had a slightly curved nucleus while those of T. mactroides had a long, prominently curved nucleus. The mitochondria were equally distributed around die centrioles in the midpiece of A. brasiliana spermatozoa, but asymmetrically in the midpiece of T. mactroides spermatozoa. There were six mitochondria and glycogen clusters in the middle piece of the T. mactroides spermatozoon. The presence of glycogen clusters and die higher number of mitochondria, in comparison with Anomalocardia brasiliana, could extend the longevity of the Tivela mactroides spermatozoa. An increase in sperm life expectancy implies in an increase in the probability of finding eggs and accomplishing fertilization. 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