A synthesis of 6a-carbaprostaglandin I₃ (2) was achieved in which the ω-chain was introduced by Wittig reaction of the optically active aldehyde (3) with the β-oxido ylid (4), followed by removal of the protecting groups to afford a chromatographically separable mixture of the 13 (E)-product (8) and its 13 (Z)-isomer (7). The geometry of the double bond was determined by direct comparison of 8 with a standard sample synthesized by an alternative route (Chart 2). Compound 8 was easily transformed into 2 by a 6-step sequence of reactions (Chart 3). By using a similar technique, (13Z)-6a-carbaprostaglandin I₃ (1A) was also synthesized from the 13 (Z)-isomer (6).