Two cases of gongylonematiasis in common marmosets of two research facilities in Germany are reported. The helminthiasis was transmitted from colony A to colony B by one infected female and within colony B by cockroaches (Blatella germanica). Four of 40 cockroaches examined in colony B were infected with rhabditiform larvae. Clinical signs of disease in the infected animals consisted of intense itching and scratching of the edematous and slightly hyperemic perioral tissues. Histologically the adult helminths lodged predominantly in the mucous membranes of the upper and lower lips; they were less frequently present in the labial cutaneous parts or in the tongue and could not be seen in the esophageal wall, bronchi, or abdominal organs. The helminthic infection probably caused minor to moderate mixed inflammatory infiltrates of the periesophageal connective tissues and intense inflammation of the deep lingual muscular tissues. Lesions of the mucosal membranes of the lips and tongue had predominant accumulations of neutrophilic granulocytes, lymphocytes, and mostly degranulated mast cells with only a few eosinophilic granulocytes. In the cutaneous part of the lips multifocal microabscesses were considered to be secondary lesions from the intense scratching.