The medium formulation and robust process modeling for PHA production by fermentation from industrial waste (ice cream residue) was studied by employing novel crossed experimental design. A crossed design, mixture design combined with process factor (fermentation time), was performed to find the optimal medium formulation and process time. The optimal settings for three major components (50.00 ml of ice cream, 8.64 ml of buffer, and 41.36 ml of distilled water) characterized by lipid (16.2 mg/ml) and % lipid (89.5%) values were found and further investigated to find robust process conditions (51.52 ml of ice cream, 7.94 ml of buffer, 40.54 ml of distilled water, and 215.37 h of fermentation time) for PHA production (16.2 mg/ml for lipid, 87.7% for % lipid) by applying propagation of error (POE). (C) 2004 Elsevier Ltd. All rights reserved.