Summary Biochemical tests were made on several strains representing each of the twelve known serotypes of Pasteurella haemolytica. The use of the fermentation reactions with arabinose and trehalose to distinguish the A and T biotypes was found to be more reliable for T than for A strains, since some strains of biotype A in all serotypes, except serotype 11, failed to ferment arabinose within 14 days. Mannose and salicin were fermented by strains of biotype T, but by only 1 (mannose) or 2 (salicin) out of 30 strains in biotype A. Lactose and xylose were fermented by A strains only. All strains reduced nitrate and gave negative reactions in the methyl-red and Voges-Proskauer tests. We are grateful for the encouragement and advice given by Dr J. T. Stamp and Mr J. G. Brotherston, Director and Deputy Director, ressectively, of Moredun Institute, where the work was performed. We are also indebted to Professor E. L. Biberstein for criticism of the manuscript