Deoxyribonucleic acids (DNAs) from an unnamed species of microorganism (bovine encephalitis isolate) previously placed in the family Brucellaceae and from representative species of this family were thermally denatured t o determine mole percentages of guanine plus cytosine (mol %G+C). The results established that DNA from the bovine encephalitis isolate, had a mol %G+C of 37.3 * 0.2, which was between values for Haemophilus influenzae and Francisella tularensis. Values for mol %G+C of the other microorganisms in this family agreed with previously reported data. A heterogenous group of microorganisms composed of six different overlapping genera and the BE1 were in the range of 3 3 t o 46 mol %G+C. Because the mol %G+C of the genera in this group did not differ statistically, the proper taxonomic position of recognized members of the family Brucellaceae was questioned.