The toxicity of a series of organo‐phosphorus compounds to the overwintering eggs ofAphis pomiDeg.,Operophtera brumata(L.) andMetatetranychus ulmi(Koch) was examined by a laboratory technique. The eggs ofM. ulmiwere highly resistant to all the compounds tested, those ofOperophtera brumatawere most susceptibleto P‐nitrophenyl diethyl thionphosphate (E 605) and those ofAphis pomitop‐nitro‐phenyl diethyl phosphate (E 600). With analogues of the last‐mentioned compound there is evidence that their toxicity toA. pomieggs is associated with the presence of electron‐attracting substituents in the benzene nucleus. A number of organo‐phosphorus compounds, including hexaethyl tetraphosphate andp‐nitrophenyl diethyl thionphosphate, were highly toxic under laboratory conditions to the active summer stages ofMetatetranychus ulmi, but the summer eggs of this mite were rather more resistant. Field trials of hexaethyl tetraphosphate andp‐nitrophenyl diethyl thionphosphate for the control of the aphidsBrachycaudus helichrysi(Kalt.),Phorodon hutnuli(Schr.) andAphis pomiDeg. showed these substances to be effective substit