The action of smoke from smouldering mosquito coils against Aedes aegypti(L.), Anopheles stephensi List. and Culex pipiens fatigans Wied. was studied. Smokes from lindane or DDT coils did not inhibit biting of guinea-pigs by Ae. aegypti or A. stephensi nor did they cause knockdown. The pyrethroids (allethrin, pyrethrins, bioallethrin and S-bioallethrin in that order) were increasingly effective in knocking down and killing C. p. fatigans and Ae. aegypti, and their bite-inhibiting activity on Ae. aegypti and A. stephensi increased in the same sequence. Pyrethrins were inferior to allethrin for knockdown of A. stephensi. Smoke from a bioallethrin coil inhibited Ae. aegypti from probing and taking blood from man. Tests in a cylinder of 0·034 m3 and a room of 25 m3 suggested approximate relative potencies of 1:2:4 for allethrin, bioallethrin and S-bioallethrin, the bite-inhibitory and knockdown actions being closely associated. Use of gauze-ended test cages reduced the range of relative potencies. The discussion suggests that the sequence of effects exerted by smoke on a mosquito entering a room is deterrency, expellency, interference with host finding, bite inhibition, knockdown and, eventually, death.