This invention relates to novel prostacyclin derivatives, their acceptable acid addition salts, solvates, hydrates and polymorphs thereof. The invention also provides compositions comprising a compound of this invention and the use of such compositions in methods of treating diseases and conditions beneficially treated by prostacyclin, and in particular those diseases and conditions beneficially treated by dilators of systemic and pulmonary arterial vascular beds or by platelet aggregation inhibitors.