The synergistic action of a juvenile hormone ana logue RO 20-3600 on allethrin against adults of house fly (Musca donestica L.) was evaluated by fitting the Hewlett's mathematical model. The mixture of allethrin with a sufficiently large ratio of RO 20-3600 was estimated to be 20.8 times as toxic as allethrin alone. This degree of the action, however, was equivalent to the result previously obtained by Nagasawa et al. in combination of allethrin and piperonyl butoxide against the same strain of house fly.