Disclosed are two new crystalline forms, δ and ε, of perindopril erbumine. Those forms are suitable as therapeutic active substances for medicaments for the treatment of cardio-vascular diseases, especially high blood pressure and heart failure. The ε crystalline form is obtained in the crystallization of perindopril erbumine at from 30 to 45° C., preferably from 34 to 45° C., from MTBE containing from 1.5 to 2.5% (v/v) water; the crystallization is advantageously carried out with stirring. If the water is then removed, advantageously by azeotropic distillation, preferably at from 35 to 37° C., and stirring is then continued for at least 15 h at from 30 to 45° C., preferably from 35 to 37° C., the ε crystalline form is converted to the δ crystalline form. The δ crystalline form can also be obtained by stirring the α or β crystalline form at from 33 to 38° C. in tert.-butyl methyl ether containing from 0.9 to 1.4% (v/v) water with seeding with the δ crystalline form. The ε crystalline form can also be obtained by stirring the α or β crystalline form at from 28 to 35° C. in tert.-butyl methyl ether containing from 0.9 to 1.4% (v/v) water with seeding with the ε crystalline form; or by stirring the α or β crystalline form at from 35 to 38° C. in tert.-butyl methyl ether containing from 1.5 to 2.0% (v/v) water.