According to the present invention there is provided a synergistic insecticidal composition containing more than one active ingredients and being harmless to environment characterized by comprising in an amount of from 0.001 to 99% by weight a synthetic pyrethroid of the Formula /I/ ##STR1## namely substantially only the 1RtransS and 1StransR entantiomer-pair /Ib/ out from the possible eight isomers--optionally in admixture with an amount of up to 100% by weight of one or more activator/s/ and auxiliary agent/s/, particularly antioxidants, stabilizing agents, wetting agents, emulsifying agents, dispersing agents, antifoam agents, diluents and/or fillers.The enantiomer-pair Ib consisting of the 1RtransS and 1StransR isomers is new and the invention also relates to the said new enantiomer-pair and a process for the preparation thereof.The advantage of the insecticidal composition of the present invention is that it is less toxical towards warm-blooded animals and useful parasites and is therefore much less harmful to the environment.