Provided is a novel ivermectin derivative and compositions for the treatment or prophylaxis of parasite infestations in mammals or birds which comprise: (A) a pharmaceutically effective amount of a 1-N-phenylpyrazole compound; (B) a pharmaceutically effective amount of an ivermectin derivative of formula (II) wherein: R.sub.14 represents --(CH.sub.2).sub.sO--Z wherein, s is 1 or 2; Y represents --CH(OR.sub.15)--, --C(.dbd.O)-- or --C(.dbd.NOR.sub.15); R.sub.15 represents hydrogen, alkyl or phenyl; and R.sub.16 represents --CH.sub.3 or --CH.sub.2CH.sub.3; Z is alkyl, alkenyl, alkynyl, acyl, alkylalkoxy, aryl, alkanoyloxy, alkoxycarbonyl, alkenoyl, alkynoyl, or aroyl.