1.1. In vivo and in vitro experiments were performed to study the decarboxylation of tetracosanoic acid in the termite Zootermopsis angusticollis. Radio-GLC of the alkanes formed from the metabolism of [15,16-3H]tetracosanoic acid both in intact animals and in cell free preparations showed that the substrate was directly decarboxylated to n-tricosane.2.2. α-Hydroxytetracosanoic acid was incorporated into hydrocarbon more efficiently than was tetracosanoic acid, suggesting that the first step in the reductive decarboxylation of very long chain fatty acids in insects involves α-hydroxylation.3.3. The decarboxylase activity was located primarily in the microsomal fraction, and was stimulated twofold by the addition of ascorbic acid. The pH optimum for the reaction was 6.0. Neither ATP, CoASH, nor reduced pyridine nucleotides increased enzyme activity.