Abstract: (+)- or (?)-erythro-Mefloquine hydrochloride can exist in four crystalline forms A, B, C and D, whereby form A is the most stable form. Form A can be directly produced in morphological forms like thick columns, cuboids, cubes and cube-like forms, which can be easily handled during processing and formulation. (+)- or (?)-eroryth-Mefloquine hydrochloride also forms solvates with acetone, methyl ethyl ketone and tetrahydrofuran.