Oxygen transfer performances in intensive microbial cultures are compared with those occurring in coalescing and non-coalescing mineral media. E. coli fed-batch cultures are carried out in a 22 L bioreactor. Biomass concentrations of 80 g(DW) L(-1) are reached, with oxygen consumption rates of up to 0.6 mol L(-1) h(-1). To achieve these high transfer performances, dissipated power e reaches 35 kW m(-3). The hold-up in the culture broth and in the corresponding supernatant matches the non-coalescing mineral medium. Oxygen transfer coefficients, K (L) a in mineral media, and K (T) in the culture broth, are compared. K (T), calculated online from a gas balance method, excesses 1 s(-1). Yet, for given values of e, K (T) is 4-8 times lower than K (L) a determined in the non-coalescing mineral medium. The cell activity modifies the chemical medium properties and reduces the oxygen transfer conductance, as in a non-coalescing ionic medium containing surfactant.