The authors report two cases of actinomycosis in children: one thoracic and the other retroperitoneal. They emphasize the difficulties of diagnosis before the stage of parietal extension with cutaneous fistula and characteristic yellow granular discharge. These difficulties are due to: The rarity of visceral actinomycosis, particularly in children. The lesion has a similar appearance to that of a tumor; an extensive pre-operative work-up is mandatory (ultrasound, computed tomogram scan, repeated ultrasound-guided needle biopsy), although this work-up may not necessarily lead to the correct diagnosis. A surgical biopsy will often confirm the diagnosis, provided the diagnosis has been previously considered. The necessity of using very specific tests for correct identification of the organism. Therefore, in a case of pseudo-inflammatory pseudotumor, visceral actinomycosis must be considered in order to guide microbiological and pathological studies, although this diagnosis is rare. Once the diagnosis has been made, prolonged treatment with penicillin is effective and complete recovery is generally obtained.