This was probably due to the substrate inhibition. Similar observations were made by other scientists. Belfaos et al., (1995) and Shaaban et al., (2012) reported that glucose and ammonium ions were inhibitory at certain levels, which affect the specific growth rate and PHB production. Inhibition by ammonium ions Heinzle and Lafferty, (1980) and substrate inhibition by carbon source Lee and Yoo, (1991) on PHB production have been reported.