The faecal enterococcal population of chickens, fed a ration containing no coc‐cidiostat or growth‐promoting antibiotic, was monitored during the first 23 d after purchase as day‐old birds. The proportion ofEnterococcus faecalis, the majority of which were resistant to streptomycin (60μg/ml) and tetracycline (30 μg/ml), fell during the first week while there was a corresponding increase in the proportion ofEnt. gallinarum. Enterococcus faecium, half of which were resistant to cephalothin (30 μg/ml), increased during the first week but thereafter rarely exceeded a quarter of the enterococcal isolates. The changes in the relative proportion of these three bacterial species was reflected by changes in the incidence of resistance to these antibiotics in the enterococcal population as