Abstract--1. The biosynthesis of hydrocarbons in the snail Cepaea nemoralis was studied after injection of the 14C-labelled precursors acetate, valine, isoleucine and palmitic acid.2. The highest incorporation was achieved with palmitic acid, although with the other precursors the hydrocarbons were also distinctly labelled.3. The composition of the hydrocarbons was determined by gas-liquid chromatography, involving a series of n-alkanes (n-C15-n-C37) as the major components (over 83 per cent).