A pesticidal compound of formula I: ##STR1## in which formula: either R.sup.1 is hydrogen and R.sup.2 represents a cyclopropyl group or R.sup.1 and R.sup.2 each represent an alkyl group, which alkyl groups may be the same or different; Ar.sub.A represents an optionally substituted phenyl or naphthyl group; Ar.sub.B represents a phenoxy, phenyl, benzyl or benzoyl-substituted phenyl group which is optionally further substituted; the configuration of the groups Ar.sub.A --CR.sup.1 R.sup.2 and --CH.sub.2 Ar.sub.B about the double bond being mutually trans. Preferably Ar.sub.A is a substituted phenyl group; more preferably being substituted at the 4-(para) position by halogen, alkoxy or haloalkyl. Further provided is a process for the preparation of a pesticidal compound of formula I in which a compound comprising a moiety ##STR2## and a compound comprising a moiety Ar.sub.B -- are reacted together forming a link --CH.dbd.C(F)CH.sub.2 -- between ##STR3## and Ar.sub.B in the compound of formula I.
Compounds of the formula ##STR1## in which Ar is substituted or unsubstituted phenyl, naphthyl, or thienyl; Z is oxygen, sulfur, or methylene; and Ar' is 2-methyl[1,1'-biphenyl]-3-yl, 3-phenoxyphenyl, 4-fluoro-3-phenoxyphenyl, or 6-phenoxy-2-pyridyl exhibit pyrethroid-like insecticidal and acaricidal activity and are relatively harmless to aquatic fauna.