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Bivalve mollusks of the family Mytilidae inhabit almost all regions of the world ocean. Representatives of the genera Mytilus and Crenomytilus that are impor� tant objects of the mariculture and commercial catch� ing, dominate in coastal waters of the Sea of Japan. In the study of reproductive strategies of mytilids, their spermatozoa were described by a number of authors [3, 8, 12–14]. The data covered, mainly, peculiarities of morphology and ultrastructure of spermatozoa and traditionally were used for decision of questions in tax� onomy of this group of mollusks. Transmission elec� tron microscopy has shown that mytilids are charac� terized by a “primitive” type of the spermatozoon, which is usually found in invertebrate animals with external fertilization [10]. Until recent time, it has been considered that only one sperm type is inherent in each species of mytilids. However, we have revealed unusual cases of the heteromorphism in the sperm of Gray’s mussel Crenomytilus grayanus (Dunker, 1853) and the Pacific blue mussel Mytilus trossulus Gould, 1850. The goal of this study was detailed analysis of the heteromorphism phenomenon in male gametes of these two species. Mature gonads of mollusks, collected in the Vostok Bay (the Sea of Japan) in July–August 2011 were used for the study. Material was collected at the Vostok Marine Biological Station of the Institute of Marine Biology. Under laboratory conditions, gonads were removed and crashed into small pieces, which were fixed at 4°C in 2.5% glutaraldehyde in 0.2 M cacody� late buffer solution (pH 7.4) for 2–4 h. Then, the spec� imens were washed in 0.1 M cacodylate buffer solution for 15–20 min. The sperm suspension was placed by a pipette onto the surface of Thermanox cover slips (Cat. no. 72280), incubated until they sedimented (for 30 min), and then dehydrated in a series of accelerat� ing concentrations of ethanol, gradually bringing to a pure acetone. After that, the specimens were critical�