Experimental infections with Loa loa were established in laboratory primates by inoculation of third-stage larvae recovered from either Chrysops silacea infected after engorging on a human volunteer with loiasis, or Chrysops atlanticus infected after blood feeding on experimentally infected primates. The third molt occurred at 16 to 20 days postinoculation of infective larvae, and the fourth at about 50 days. Early larval development was characterized by rapid growth and marked differentiation of the reproductive systems. Differences in the body's growth rate between the sexes were seen as early as 30 days and was especially marked by 60 days. Females grew more rapidly, and for a longer time than males. Males reach maximum size by 120 days, females not until about 300 days. Female worms were inseminated prior to day 90 developing eggs filled the uteri by 120 days, and microfilariae appeared in the peripheral blood at approximately 150 days. Morphological features of the larval and adult stages of human Loa loa are described.