An implantable prosthetic aortic valve comprising: a frame having three angularly spaced commissure attachment posts and a plurality of angularly spaced, axial struts that are interconnected by at least three rows of circumferential struts including an inflow row of circumferential struts adjacent the inflow end of the frame, an outflow row of circumferential struts adjacent the outflow end of the frame, and an intermediate row of circumferential struts positioned axially between the inflow and outflow rows of angled struts along the length of the frame, each row of circumferential struts including struts arranged in a zig-zag pattern extending around the circumference of the frame, wherein pairs of adjacent struts of the intermediate row of circumferential struts and pairs of adjacent struts of the outflow rows of circumferential struts form cells; an annular skirt member positioned inside of and secured to the frame; and a leaflet structure comprising three leaflets made of natural material, the leaflet structure having a scalloped lower edge portion secured to an inner surface of the skirt member and three commissures formed at adjacent sides of the leaflets, each commissure being attached to one of the commissure attachment posts, the skirt member disposed between the frame and the leaflet structure; characterized by a reinforcing strip, separate from the skirt member, secured to an inner surface of the scalloped lower edge portion of the leaflet structure, wherein each leaflet has an upper edge, a curved lower edge and two side flaps extending between respective ends of the upper edge and the lower edge, wherein each side flap is secured to an adjacent side flap of another leaflet to form the commissures of the leaflet structure, and wherein the radially collapsible and expandable annular frame is made of a nickel-cobalt-chromium alloy. a frame having three angularly spaced commissure attachment posts and a plurality of angularly spaced, axial struts that are interconnected by at least three rows of circumferential struts including an inflow row of circumferential struts adjacent the inflow end of the frame, an outflow row of circumferential struts adjacent the outflow end of the frame, and an intermediate row of circumferential struts positioned axially between the inflow and outflow rows of angled struts along the length of the frame, each row of circumferential struts including struts arranged in a zig-zag pattern extending around the circumference of the frame, wherein pairs of adjacent struts of the intermediate row of circumferential struts and pairs of adjacent struts of the outflow rows of circumferential struts form cells; an annular skirt member positioned inside of and secured to the frame; and a leaflet structure comprising three leaflets made of natural material, the leaflet structure having a scalloped lower edge portion secured to an inner surface of the skirt member and three commissures formed at adjacent sides of the leaflets, each commissure being attached to one of the commissure attachment posts, the skirt member disposed between the frame and the leaflet structure; characterized by a reinforcing strip, separate from the skirt member, secured to an inner surface of the scalloped lower edge portion of the leaflet structure, wherein each leaflet has an upper edge, a curved lower edge and two side flaps extending between respective ends of the upper edge and the lower edge, wherein each side flap is secured to an adjacent side flap of another leaflet to form the commissures of the leaflet structure, and wherein the radially collapsible and expandable annular frame is made of a nickel-cobalt-chromium alloy.