Six hundred larvae of all instars of Aedes aegypti Linnaeus, 1762 (Diptera: Culicidae) were experimentally infected with larvae of Paragordius varius (Leidy, 1851) (Gordiida: Nematomorpha). All A. aegypti larvae were examined under a light microscope in order to assess the percentage of parasitized larvae, the number of P. varius larvae per A. aegypti larva and their location (thorax and/or abdomen). ANOVA was used for statistical testing of treatment effects. Data of the number of P. varius larvae per A. aegypti larva were log transformed and analyzed by two-factors ANOVA (instars: 1^st, 2^nd, 3^rd, 4^th; position: thorax and abdomen). Means were compared by Tukey test and the significance level was set at P< 0.05. The results obtained showed that A. aegypti larval mortality, at 2h 30min after infection, was higher in parasitized larvae (42.5 %) than in the control group (3 %). First and 2^nd instar larvae were the most susceptible to parasitism (52.15% and 46.67 % respectively). More than 90% of each instar A. aegypti larvae were parasitized and the percentage parasitism did not differ between instars. More parasitic larvae were found in 1^st and 2^nd instar A. aegypti larvae than in 3^rd and 4^th instars. Paragordius varius parasitize predominantly the abdomen. 9.1% 3^rd instar and 68.4% 4^th instar Aedes aegypti larvae induced an internal defense reaction (IDR). No infection was registered in the control group.