A suspension concentrate of Bacillus thuringiensis serotype H-14 formulation (Teknar, SAN 402-1 SC) was tested against laboratory reared late third/early fourth instar larvae of Mansonia uniformis as well as naturally occurring Mansonia larvae using Hudson knapsack sprayers on small plots in swampy ditches on Penang Island, Malaysia. Six dosages ranging from 1.1 to 11.40 kg/hectare were used in two experiments. Mean dosage/response values at the 50% level for the introduced and natural population were 0.66 and 1.19 kg/hectare, respectively, whereas, the mean dosage/response values at 95% level were 11.02 and 25.98 kg/hectare for the introduced and natural population, respectively. Higher dosages of the B. thuringiensis H-14 formulation were needed to achieve control of the Mansonia larvae when compared with other vector mosquitoes. The heterogeneity of the response of Mansonia population towards B. thuringiensis H-14 was also observed. The comparable dosage/response values for introduced and natural populations suggest that caged introduced populations can be used as a bioassay method for Mansonia larvae in the field.