Provided for in the instant application are two additional forms of rifaximin, namely rifaximin polymorphic forms APO-III and APO-IV. Also provided are allegedly novel processes for preparing the previously disclosed rifaximin polymorphic forms APO-I and APO-II. Rifaximin is a non-aminoglycoside antibiotic that has previously been found to be useful for the treatment of traveller's diarrhea caused by Escherichia coli bacteria, as well as in the treatment of irritable bowel syndrome, diverticular disease, hepatic encephalopathy, pyogenic skin infections and as an antibacterial prophylactic prior to colon surgery.