Abstract1‐Alkylcyclopropanecarboxylic acids are synthesised by thermolysis of 1‐pyrazolines prepared by 1,3‐dipolarcycloadditions of diazoalkanes. The insecticidal and acaricidal activities of 14 pyrethroid esters of these acids are considered in relation to the influence of steric and metabolic factors on activity. 1‐Alkyl compounds are less active than esters with no alkyl group at position 1, contrary to expectations, and also less active than the corresponding 2‐alk