Aims: The study investigated the physiology of Amaricoccus kaplicensis to determine whether it could outcompete polyphosphate accumulating bacteria in activated sludge systems removing phosphorus, by preferentially assimilating substrates in the anaerobic stages of these processes. Methods and Results: The storage processes were investigated under anaerobic, anoxic and aerobic conditions in both batch and periodically fed cultures in an aerobic sequencing batch reactor (SBR). Amaricoccus kaplicensis showed a high capacity for storing aerobically large amounts of acetate as poly beta -hydroxybutyrate (PHB) at high rates. However, no acetate assimilation under anaerobic conditions and very slow assimilation under anoxic conditions could be detected. Conclusions: Amaricoccus kaplicensis in pure culture does not behave as polyphosphate accumulating bacteria competitor; therefore it is difficult to understand why anaerobic/aerobic systems often contain such large numbers of Amaricoccus cells. Significance and Impact of the Study: Amaricoccus kaplicensis is probably not responsible for the failure of activated sludge systems removing phosphorus, and other organisms capable of anaerobic substrate assimilation should be sought.