Some illustrative results are presented for a 3 m(3) triple-impeller stirred reactor producing tylosin (using Streptomyces fradiae) being fed with air and a concentrated aqueous ammonium nitrate solution. The analysis is performed for a 3 x 2(10 x 10) configuration of a networks-of-zones model comprising 600 zones. The gas-liquid flows, giving rise to circulation and bubble dispersion, are calculated assuming that bubbles move independently at their rise velocity without distorting the liquid Rows bring generated by tilt: impellers. In this way the gas hold-up distribution in 3-D is obtained from simple continuity principles. Mass transfer and a 2nd order bioreaction may then be combined with gas-liquid flow to predict the spatial distribution of the (pseudo-stationary) concentration fields. The observed overall gas voidage. overall oxygen mass transfer and a locally measured dissolved oxygen level are all exactly matched by the simulations. The predicted segregated concentration fields show remarkably complicated patterns. Micro-organisms following stochastic paths around the vessel will experience wide variations in chemical environments.