The effect of diethylcarbamazine (DEC) on infective larvae and immature worms of Brugia pahangi was studied in vitro. The in vitro culture of larvae was done using the technique of Mak et al. (1983). In control cultures, most larvae remained alive for 14 days; over 50% survived until day 22 of cultivation. The addition of DEC did not affect the life span of the larvae. Among those which survived for 22 days in control cultures, 77.8% reached the fourth stage, their length being 2908.2 +/- 453.2 microns. When DEC was added to a final concentration of 0.1 mg/ml, the percentage of larvae attaining the fourth stage was reduced (42.9%) and their growth retarded; the length of the fourth-stage larvae was 2548.4 +/- 414.0 microns. The addition of 1.0 mg/ml DEC completely arrested the growth and development of the larvae.