In this study, changes in microbial diversity and polyhydroxyalkanoate storage ability of activated sludge under aerobic dynamic feeding conditions were investigated. Two sequencing batch reactors were operated with and without nitrogen limitation, by applying a moderate sludge retention time. Polymer storage abilities of the biomasses were improved significantly under dynamic conditions, in terms of specific polymer storage rate, polymer storage yield and polymer content of activated sludge. Moreover, aerobic dynamic feeding conditions together with nutrient limitation further improved the storage ability of the mixed population. Polymer storage yields of the biomass enriched under nitrogen-sufficient and nitrogen-deficient conditions were 0.43 and 0.61 Cmmol PHA/Cmmol substrate, respectively. This study also contributes to the knowledge of activated sludge microbiology, providing detailed information about temporal changes in community structure under dynamic conditions. Microbial community structure was determined by 16S rDNA clone library construction. Also changes in communities under different operating conditions were monitored by DGGE analysis based on bacterial 16S rDNA. The beta subclass of Proteobacteria was the most abundant phylum in both reactors during the operation periods. Changes in the community structure occurred in terms of relative abundance of the operational taxonomic units (OTUs) rather than the OTU types present in the system.