One strain ofYersinia enterocolitica and one strain ofY. intermedia were grown in peptone water at 25 or 37 °C, or in ground water at 15 °C. Similar growth rates were observed when these strains were cultivated separately in the same media and at the same temperature. Mixed cultures at 37 °C displayed equivalent growth rates. In contrast, mixed cultures incubated at 15 or 25 °C were regularly unfavourable toY. enterocolitica, whereas they did not modify the growth ofY. intermedia. A bacteriophage active onY. enterocolitica and not onY. intermedia was characterized from the filtrate of mixed cultures at low temperatures. This phage produced by the lysogenicY. intermedia strain might be a potential factor responsible for the inhibition ofY. enterocolitica, since no additional antibacterial factor or nutritional competition betweenY. intermedia andY. enterocolitica were found in the mixed cultures.