Actinobacillus is a genus of Gram-negative, immotile and nonspore-forming, oval to rod-shaped bacteria occurring as parasites or pathogens in mammals, birds, and reptiles. It is a member of the Pasteurellaceae family. The bacteria are facultatively aerobic or anaerobic, capable of fermenting carbohydrates (without production of gas), and of reducing nitrates. The genomic DNA contains between 40 and 47 mol % guanine plus cytosine.
An analysis of A. actinomycetemcomitans indicated it was monophyletic with Haemophilus aphrophilus and H. segnis, and it was proposed that they be reclassified as a new genus, Aggregatibacter (from the Latin, "aggregare", meaning "to come together").