BACKGROUND: The aim of this research was the optimization of poly(3-hydroxybutyrate) - P(3HB) - production in submerged cultures of Bacillus megaterium in a mineral medium using sucrose as carbon source and nitrogen as the limiting substrate. Small-scale experiments were carried out in shake flasks at 30 degrees C and 160 rpm in order to evaluate the best initial sucrose concentration and carbon: nitrogen ratio to maximize biomass accumulation and biopolymer production. An objective function in terms of residual sucrose and P(3HB) production was proposed in order to optimize the amount of carbon source used and the production of P(3HB). RESULTS: High production of P(3HB) was obtained, with approximately 70% (CDW) accumulation in cells without nitrogen limitation and strongly correlated with the pH of the culture. Scaling-up the system to cultures in a bioreactor, with or without pH control, a reduction of P(3HB) accumulation (around 30% CDW) was observed when compared with shaker cultures, suggesting a possible role of oxygen limitation as a stress signaling for P(3HB) synthesis. CONCLUSIONS: Results of our experiments showed that Bacillus megaterium was able to produce P(3HB) at one of the highest production rates so far reported for this bacterium, making this microorganism very interesting for industrial applications. Comparisons of shaker and bench-scale bioreactor experiments show both the importance of pH and aeration strategies. It is likely that complex aeration strategies linked to cell metabolism will be necessary for further developments using this bacterium. (C) 2009 Society of Chemical Industry