Crassostrea specimens recognized in the Pleistocene sediments of Ecuador represent the first fossil thick-shelled species of this genus in the Pacific South America. The specimens come from the Pleistocene Jama Formation exposed along the coastal cliffs of central Ecuador in the Esmeraldas-Caraquez Basin. The middle stratigraphie unit of this formation (the Punta Ballena Member) is about 100 m thick and composed of eight superposed, unconformity-bounded depositional sequences (PB1/PB8) with a highfrequency cyclicity (40 ky The Crassostrea specimens collected in the middle of the PB5 fining-upward sequence may belong to a new species of this genus. The oyster concentration is a lens-shaped bank up to 60-70 cm thick and a few meters long where thick-shelled, large-sized, and elongated individuals in general good state of preservation are embedded in a soft, muddy to fine-grained sandy matrix. They display a high degree of packing and locally a bioclast-supported biofabric. Some specimens consist of disarticulated valves oriented both convex-up and convex-down whereas conjoined shells are predominant in number and arranged with different orientations from nearly horizontal to oblique. A few complete shells show a vertical position with the commissural plane more or less perpendicular to the bottom and the ligamental area pointing downwards. Fragmentation and abrasion are scarce, signs of encrustation and bioerosion are rare and occur only on the external surface of few valves. SEM investigation on the shell micro structure points out a thin outer prismatic layer composed of compact, closely joined calcific prisms, irregularly polygonal in section and obliquely or nearly perpendicular oriented to the outer surface (regular simply prismatic, RSP The underlying shell shows alternating thin and thick layers. The thin layers consist of continued sheets oriented nearly parallel or oblique to the valve surface and arranged as "tiles of a roof (regularly foliated, RF) whereas the thick layers display a severely re-crystallized structure originally composed of parallel/ subparallel calcific blades interconnected by smaller calcific leaflets. The large amount of void spaces interlocked between blades and leaflets accounts for the original "chalky" appearance of the porous structure (crossed lamellar, CL The elongated and flat shape of the valves along with the elongate and triangular shape of the ligamental area are typical morphological attributes of the "secondary soft-bottom dwellers" with vertical life posture. Porous chalky deposits have been interpreted as an adaptive strategy to prevent the sinking in the soft bottom, but they may have been also used as a mechanism to increase shell thickness deterring prédation by fully marine organisms. We infer that individuals lived as seminfaunal with a vertical posture of life within a soft bottom of shallow-water environments (lower shoreface to inner shelf) and interested by dislodging, before or after death, due to storm-induced wave/current action.