Blood surveys conducted among the aboriginal Indians (Amerinds) in the interior of Guyana revealed that Dipetalonema perstans or Mansonella ozzardi, or both, occurred in approximately 12% of the persons examined, D. perstans being more common and widely distributed. It was found in Amerinds in all of the districts surveyed and in persons of other than aboriginal stock living in the interior of the country. M. ozzardi, in contrast to D. perstans, had a low prevalence and a very limited geographic distribution. It was found among Amerinds in the Pakaraima Mountains of the Mazaruni-Potaro and Rupununi Districts and in the North West District, but was absent from many areas in which it had been found at the turn of the century. M. ozzardi occurred more frequently mixed with D. perstans than as a pure infection. The geographic distribution of each species in the various administrative districts of the country is presented.