The invention provides new crystalline forms of 7-[3,5-Dihydroxy-2-(3-hydroxy-5-phenyl-pent-1-enyl)-cyclopentyl]-hept-5-enoic acid. This compound is commonly referred to as “bimatoprost acid.” The novel crystalline forms are designated forms I, II, and III. The invention crystalline forms are useful for solid ocular implant formulations, utilized in the treatment of various ocular conditions, such as, for example, ocular hypertension. In addition, invention crystalline forms are useful for solid or semisolid dosage formulations used to treat ocular hypertension.