A blood or yeast dependent pleomorphic Gram-negative bacillus was isolated from the pneumonic lung of a suckled calf which died suddenly. The organism was biochemically similar to American strains of Haemophilus somnus and was shown to be serologically similar by rapid slide agglutination, tube agglutination and micro-complement fixation tests. The possible importance of this organism in disease syndromes in cattle in the United Kingdom is discussed.