To our knowledge, we report the first case of transdiaphragmatic Actinobacillus actinomycetemcomitans infection in which the patient presented with an abdominal tumor. The clinical manifestation of this infection was similar to that of Actinomyces israelii infection and was initially misinterpreted as a malignancy. The actual diagnosis was made following prolonged culture of the biopsy specimen of the involved tissue. The pathogen was susceptible to penicillin, ampicillin, tetracycline, cephalosporins, metronidazole, aminoglycosides, and quinolones. The patient was treated with a combination of amoxicillin and norfloxacin, and 3 months later his condition had clinically improved.