The present invention provides a class of substituted bicyclooctanes of formula ##STR1## wherein R is a C.sub.2-10 non-aromatic hydrocarbyl optionally substituted by cyano, halo, C.sub.1-4 alkoxy, C.sub.1-4 haloalkoxy or S(O).sub.m R.sup.3 ; R.sup.1 and r.sup.2 are independently hydrogen, halo, a C.sub.1-3 aliphatic group optionally substituted, S(O).sub.m,R.sup.4, or R.sup.1 and R.sup.2 and the carbon atoms to which they are attached form a C.sub.5-7 carbocyclic ring optionally substituted by halo, or a C.sub.1-3 aliphatic or alkoxy group; A-X is a group: (i) A'C.tbd.CX in which A' is a C.sub.3-6 alkyl, alkenyl or alkynyl or a C.sub.3-10 cycloalkyl or cycloalkenyl group all optionally containing one to three hetero atoms. (ii) BX in which B is a group (CH.sub.2).sub.2 O or (CH.sub.2).sub.2 S(O).sub.n or a C.sub.2-6 aliphatic chain optionally containing one or two oxygen or sulphur atoms and/or double bonds and optionally further substituted; X is hydrogen, halo, SiR.sup.5 R.sup.6 R.sup.7 or SnR.sup.5 R.sup.6 R.sup.7 or CR.sup.8 R.sup.9 R.sup.10 ; Y and Y.sup.1 are independentaly oxygen or S(O).sub.n' and Z is CH.sub.2 S(O).sub.n " provided that when A does not contain a C.tbd.C fragment, X is a group CR.sup.8 R.sup.9 R.sup.10. The compounds have valuable pesticidal activity, particularly against arthropods and helminths. Pesticidal formulations containing the compounds of formula (I), their use in the control of pests and methods for their preparation are also disclosed.