This article provides information on the role and significance of laboratory testing in the evaluation of the dog or cat from which a potentially pathogenic bacteria is isolated. Salmonellosis, Campylobacteriosis, clostridial-induced disease, and Helicobacteriosis are all discussed from the clinical presentation of the patient to the interpretation of any laboratory testing. The diagnosis of bacterial infections of the gastrointestinal tract requires the clinician to evaluate the infectious agent, the host animal, and the effects of environmental stresses. History, carrier states, physical examination, and laboratory tests are discussed in the context of the differential diagnosis of gastroenteritis.