Abstract(S)‐Fenvalerate (α‐cyano‐3‐phenoxybenzyl) (S)‐2‐(4‐chlorophenyl)‐3‐methylbutyrate, a novel insecticide, was labelled with carbon‐14 individually at the carbonyl (1a), the cyano (1b) and the alcoholic methyne (1c) carbons for use in metabolic studies. Isopropylation of 4‐chlorophenylacetonitrile‐14C with isopropyl bromide followed by hydrolysis gave racemic 2‐(4‐chlorophenyl)‐3‐methylbutyric‐1‐14C acid, which on optical resolution with (−)‐1‐phenylethylamine gave the (S)‐acid. Esterification of the (S)‐acid with α‐cyano‐3‐phenoxybenzyl bromide gave 1a in 18% yield from potassium cyanide‐14C. Esterification of α‐cyano‐14C‐3‐phenoxybenzyl alcohol with (S)‐2‐4‐chlorophenyl‐3‐methyl‐butanoyl chloride (9) gave 1b in 65% yield. Condensation of 3‐phenoxybenzaldehyde‐(carbonyl‐14C), prepared from 3‐phenoxybenzoic‐14 C acid, with 9 and sodium cyanide gave 1c in 54% yield from barium carbonate‐14C.