This invention relates to a novel crystalline polymorph of hexyl-5-aminolevulinate hydrochloride, referred to hereinafter as Form C, which is useful as precursor of a photo sensitizer in the photodynamic diagnosis and treatment of metabolically active cells related to diseases such as cancer of the uterus, cervix, vagina, rectum, colon, lower gastrointestinal tract; infection associated with cancer caused by human papilloma virus; non-cancerous lower gastrointestinal tract a human; inflammatory bowel disease; ulcerative colitis; Crohn's disease; inflammatory bowel syndrome; dysplasia of the female reproductive system; anus; penis; rosacea; and acne and which is suitable for use as the active ingredient of a commercial pharmaceutical product. The invention relates further to methods of preparing Form C and using it in the described pharmaceutical compositions.