The production of polyhydroxyalcanoates (PHAs), which are biodegradable plastics, was studied using milk whey and dairy wastewater activated sludge to define a suitable C/N ratio, the pre-treatments required to reduce the protein content, and the effect of pH correction. The results show good production of PHAs at a C/N = 50 and without pH correction. The use of dairy wastewater activated sludge has the advantage of not requiring aseptic conditions. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.