Sustained release casein-chitosan microspheres containing diltiazem hydrochloride (DTZ) were prepared with colloidal coacervation technique in a completely aqueous environment. The interaction between chitosan solution in dilute acetic acid (5% v/v) and casein solution in 0.5 M sodium hydroxide was the basis for the microspheres formation. Formaldehyde was used for the surface hardening of the droplets by cross-linking and thus fixing the shape and surface morphology of the formed microspheres. The entrapment efficiencies of the microspheres were variables (14.5-53.7%) depending on the preparation conditions. The prepared microspheres exhibited an angle of repose values between 31.9-42.0 degrees indicating good free flowing nature, whereas DTZ powder as such was non-flowable. The dissolution profiles of DTZ from casein-chitosan microspheres showed retarded release pattern of the drug into distilled water. Casein and chitosan concentrations, initial drug concentration and stirring time were found to be the main parameters that affect the properties and the performance of the prepared microspheres. The retarded release of DTZ was increased by increasing casein concentration, and stirring time. On the other hand, increasing chitosan concentration and using high initial drug loading showed a fast drug release.