The present invention relates to insecticidal compositions of low toxicity to fishes and mammals characterized by containing m-(p-bromophenoxy)-.alpha.-cyanolbenzyl trans- or trans-rich-2,2-dimethyl-3-(2,2-dichlorovinyl)cyclopropanecarboxylate of the formula (I), ##STR1## and at least one of the organo-phosphoric esters of the formula (II), ##STR2## wherein R.sub.1 and R.sub.2 are each a C.sub.1 -C.sub.3 alkyl group, X.sub.1, X.sub.2 and X.sub.3 are each an oxygen or sulfur atom, Y.sub.1 and Y.sub.2, which may be the same or different, are each a C.sub.1 -C.sub.3 lower alkyl, methylmercapto, cyano or nitro group or a halogen atom, Z is a carbon or nitrogen atom and n is 0 or 1, in a ratio of 1 to 1-100 (part by weight) of the former to the latter, and insecticides obtained by mixing said compositions with a carrier or a diluent and if necessary an additive. The term "trans-rich" shows the ratio of trans/cis is not less than 75/25.