Precipitation of CaCO3 by different Yersinia strains on B-4 agar at 25 and 37 degrees C was studied. The 185 Yersinia strains examined produced crystals at 25 degrees C, but only 121 strains produced crystal at 37 degrees C. Strains of serotype 0:9 produced crystals at 37 degrees C, whereas strains of serotype 0:3 failed to do so. The calcium carbonate precipitation test at 37 degrees C should enable a differentiation between the two serotypes incriminated in human pathology in Europe.