Abstract Two insecticidal polyhalogenated monoterpenes, aplysiaterpenoid A and telfairine were isolated from the red alga Plocamium telfairiae. Both compounds showed strong insecticidal activities against the German cockroach ( Blatella germanica) and mosquito larvae ( Anopheles gambiae). The dieldrin-resistant strain of mosquito larvae ( A. gambiae) which exhibited the significant cross-resistance to ( 1246 35 )-1,2,3,4,5,6- hexachlorohexane (γ-BHC) also revealed weak but definite cross-resistance to aplysiaterpenoid A and telfairine. The electrophysiological symptom that telfairine increases the frequency of spontaneous discharge and prolongs the synaptic after-discharge of the central nervous system of the German cockroach ( B. germanica) in the same manner as γ-BHC indicated that telfairine also has some excitatory effect on the central nervous system. The molecular topographical studies by semiempirical molecular orbital calculation about these insecticidal monoterpenes were carried out in terms of their electrostatic similarity to picrotoxinin and extending volumes from the surface of picrotoxinin molecule. The result suggested that the orientations of the steric bulky regions and electronegative centers in aplysiaterpenoid A and telfairine could satisfy the geometrical requirement for interacting with the picrotoxinin receptor as well as γ-BHC, while the large extending volumes of both compounds led to the loose fitting into the receptor. Accordingly, we propose that two insecticidal monoterpenes from marine alga seem to interact with the picrotoxinin receptor, indicating that their mode of action is cyclodiene-type.