Through preliminary experiments, an effective method for radiolabeling Brugia malayi-infected mosquitoes in order to produce labeled infective Brugia larvae was developed. Starting on the 6th day after the infective blood meal, mosquitoes were fed a 7% sucrose solution containing 100 µCi/ml 75Se-L-methionine for 5 days. Infective larvae, retrieved 2 days after this labeling period, averaged 381 ± 136 counts/min. Jirds were infected with these infective, labeled larvae either by allowing infected mosquitoes to feed on uninfected jirds for 30 min or by inoculating jirds subcutaneously in the groin with washed larvae recovered from mosquitoes. Jirds were killed at various times after infection and were sliced into approximately 0.5 mm thick sagittal sections, which were dried and placed on X-ray film. Autoradiograms were developed after 30–60 days at 5°C. In a sample of 26 inoculated jirds, approximately 30% of the infecting larvae could subsequently be accounted for as Ag° foci on autoradiograms. The Ag° foci representing larvae were apparent up to 2.5 weeks after infection. In jirds infected by mosquito feeding, the Ag° associated with the feeding site persisted for more than 6 weeks after infection.