Examination of 75 blood samples (Knott preparation) collected in Puerto Inírida, Coco, and Pajuil, in the Comisaría del Guainía, Colombia, disclosed 26 microfilaria carriers. Eighteen persons harbored only Mansonella ozzardi microfilariae, three were infected with M. ozzardi and Dipetalonema perstans and five harbored only D. perstans. M. ozzardi infections were found in whites, and in Indians belonging to the Curripaco, Puinave, Tukano, Guanano and Saliva tribes, but D. perstans was found only in the Curripaco Indians. Numbers of circulating microfilariae (mf) were low, 73% of the carriers had less than 200 mf/ml of blood: persons who harbored only D. perstans had less than 310 mf/ml. These results confirm the presence of D. perstans in Colombia, and suggest that its prevalence and distribution in the Comisaría del Guainía and neighboring areas may be far greater than has been hitherto suspected.