A parallel dietary administration of toyocerin and colistin to piglets over a post-weaning period caused an increase in feed consumption compared with the controls. In the experimental group, substantially higher feed consumption per kg of weight gain was found. The parallel administration of colistin caused selective advantage for the propagation of the probiotic strain Bacillus toyoi and the occurrence of haemolytic strains of Escherichia coli in the experimental group was only sporadic, compared withthe controls.