A pasteurella pneumotropica like bacterium was isola ted from K.M mice colony maintained in barrier system by regular surveying, whos e phenotype and genotype have been investigated with tests available in our labo ratory. This isolate can not be distinguished from pasteurella pneumotropic in m orphology and culture characteristics such as growth and nutrition requirements. Critical biochemical tests show that the strain does not agree with the pasteur ella pneumotropic reference, while it is able to agglutinate with antiserum agai nst pasteurella pneumotropica. The chromosomal DNA G+C mol% value of the strain indicates that it can not be excluded from pasteurellae. Comparing systematic bi ochemical results with what R.boot reported in 1994,suggests that this isolate i s more like those of pasteurella sp group rather than the pasteurella pneumotrop ica Jawetz or Hely biotype, further investigation should be made in order to hav e a definite answer.