Polyhydroxyalkanoates are polyesters insoluble in water, synthesized by microorganisms and accumulated intracellular as storage materials. They are marked by high degree of polimerization, non-toxicity, wide range of application and biodegrability. Until now, there have been applied mainly pure microbial cultures for industrial PHAs production. Currently, for that purpose there are conducted intensive investigations on use of mixed microbial cultures (activated sludge). The microorganisms of activated sludge accumulate polyhydroxyalkanoates as storage materials when availability of organics as electron donors and oxygen or nitrate (electron acceptors) is distributed in time and space or when the sludge is submitted to consecutive periods of external substrate accessibility and unavailability. The process efficiency depends on feed composition and reactor operating conditions. Nowadays, there are two types of reactor operational strategies. In municipal wastewater treatment plants, a two-stage process is used: 1 ? selection of excess sludge to specific type of substrate (molesses, whey) in SBR, and 2 ? PHAs production in batch reactor. In wastewater treatment plants from agricultural and food industry, acidogenic fermentation is introduced as an additional stage, where the organic wastes are converted into a mixture of organic acids, and then used as a substrate for PHAs storage.