This paper describes a lab-scale experimentation carried out to study enhanced biological phosphate removal (EBPR) in a sequencing batch reactor (SBR). The synthetic feed used was based on peptone and glucose as organic substrate to simulate the readily biodegradable fraction of a municipal wastewater (Wentzel et al., 1991). The experimental work was divided into two runs, each characterized by different operating conditions. The phosphorus removal efficiency was considerably higher in the absence of competition for organic substrate between P-accumulating and denitrifying bacteria. The activated sludge consisted mainly of peculiar microorganisms recently described by Cech and Hartman (1990) and called “G bacteria”. The results obtained seem to be inconsistent with the general assumption that the G bacteria are characterized by anaerobic substrate uptake not connected with any polyphosphate metabolism. Supplementary anaerobic batch tests utilizing glucose, peptone and acetate as organic substrates show that the role of acetate in the biochemical mechanisms promoting EBPR may not be so essential as it has been assumed till now.