In vitro cultivation of Brugia pahangi and subperiodic Brugia malayi one-day old larvae to infective stage larvae (L3) within thoraces excised from Aedes aegypti (Black Eye, Liverpool) and Anopheles quadrimaculatus was attempted. The mosquito thoraces were excised under aseptic conditions, 24 h after a blood meal on either B. pahangi- or B. malayi-infected jirds. The excised thoraces were washed aseptically and inoculated into a diphasic media. A nutrient agar base was overlaid with either Grace's insect cell culture medium or Schneider's Drosophila medium or a mixture (1:1) of these two media. Each overlay medium contained a 1 x concentration of antibiotic/antimycotic mixture plus 20% fetal bovine serum. The excised thoraces provided the intracellular milieu for development of Brugia larvae. In Grace's or Schneider's insect tissue culture medium alone, the filaria larvae of both species developed only to the second larval stage after 12 days; whereas, in a mixture (1:1) of Grace's and Schneider's media, some one-day old larvae of both Brugia species developed to the infective larval (L3) stage after 12 days. However, large numbers of both species of larvae developed to the infective larval stage when, prior to providing an infective blood meal, the mosquitoes of both species were fed 1 x concentration of antibiotic/antimycotic mixture in a 10% sucrose solution containing 0.1% p-aminobenzoic acid for 6 days. These results showed for the first time that if one-day old Brugia larvae are confined intracellularly in excised thoraces, they can then develop in insect tissue culture media without adding a feeder layer of mosquito cells or conditioning the media with mosquito cell lines.