Polyacrylamide-co-acrylic acid (PA) added to shake flask cultures of Acetobacter xylinum at concentrations up to 3 g dm(-3) resulted in increased production of bacterial cellulose. For PA concentrations of 0-3 g dm(-3), 7-day cellulose production rose monotonically from 2.7 +/- 0.8 to 6.5 +/- 0.5 g dm(-3) at a shaker speed of 175 rpm, and from 1.7 +/- 0.01 to 3.7 +/- 0.5 g dm(-3) at shaker speed of 375 rpm. Addition of PA also changed the morphology of the biomass from amorphous/stringy forms to spheroidal particles with diameters less than or equal to2 mm. Similarly, bioreactor cultures grown in the absence of PA formed long fibrous masses which deposited on the internals, while those grown in the presence of 1-2 g dm(-3) PA formed small discrete particles with diameters less than or equal to0. 1 mm. Tests performed with I and 2 g dm(-3) PA, and stirrer speeds from 500 to 900 rpm, appeared to give the highest cellulose concentration of 5.3 +/- 0.7 g dm(-3) in 64-68.5 h in the presence of 2 g dm(-3) PA at 700 rpm, although this value was statistically indistinguishable from that obtained at 1 g dm(-3) PA and 900 rpm. A qualitative model is proposed to describe the mechanisms by which PA affects biomass morphology, resulting in its advantageous formation as small, dispersed, spheroidal pellets. Quantitative analysis of the results gave inverse correlations between both the fraction of fructose carbon going to cellulose synthesis and the specific fructose consumption rate, and the maximum cellulose concentration and the fraction of fructose carbon going to by-product formation. Since cellulose yield was almost universally improved by higher polyacrylamide concentration, it appears likely that increased viscosity reduces fructose uptake rate by limiting mass transfer. (C) 2003 Society of Chemical Industry.