The current study is an attempt to link the morphological and EBPR-related physiological traits of a propionate-fed aerobic granular biomass with the EBPR performance of the system. A lab-scale SBR was started up by the aerobic granular EBPR biomass taken from an acetate-fed SBR and the inoculum was acclimated to propionate as the sole C-source. Mechanical-mixing applied during 2-h of anaerobiosis at the head of the cycle was the main source of shear (v(SMix); 7.6 cm/s), thus hydraulic selection pressure, and feeding the system under anaerobic conditions for 1 h at the head of the cycle, as well as supplying ortho-P in the influent (COD:P=12.8 mg COD/mgPO(4)-P) were the main metabolic-selection pressures ensuring the dynamic formation, maintenance, and stability of the aerobic semi-granular EBPR biomass (d(gra); 1.41 mm) with superior settling properties and compactness (SVI <= 50 mL/g), and promoting the selection of anaerobically C-storing (>95% anaerobic COD-removal), aerobically slowly growing and Premoving organisms -the PAOs- in the system, resulting in an acceptable level of EBPR performance (78% EBPR efficiency). The biomass was considerably diverse with various morphotypes being present (rods, filaments, tetrads/sarcina-like cells, coccoid-clusters, diplo-coccoids, and elongated rod-shaped cells unique for the system). Phenotypic characterization via chemical-staining and conventional light microscopy revealed the presence of the PAOs cycling their intracellular poly-P and PHB inclusions between the anaerobic and aerobic phases. Tetrads/sarcina-like cells (TFOs) resembled the GAOs morphologically but not phenotypically. Elongated rod-shaped cells (PUBs), which had the ability to anaerobically utilize propionate and store it as PHB, were speculated to be either GAOs or functionally less efficient PAOs. Microbiological observations were qualitative, rather than quantitative, yet they found to correlate to an extend with the observed biochemical performance of the system.