SUMMARY: Several mycoplasmas isolated from pigs and designated b strains were previously found to be different from Mycoplasma hyorhinis and M. granularum, and to comprise four distinct serotypes. In the present study three of these serotypes were identified as M. laidlawii, M. gallinarum and M. iners, respectively. One serotype represented by mycoplasma B3 could not be identified. The isolation of avian serotypes from pigs is discussed in the light of host specificity of mycoplasmas.