The <i>de novo</i> formation of long-chain fatty acids and the reduction of long-chain fatty acids to the corresponding fatty alcohol were studied in the calanoid copepod <i>Calanus finmarchicus (Calanus</i>) and the fresh-water fish, <i>Trichogaster trichopterus</i> (the gourami). <i>De novo</i> biosynthesis of fatty acids by the enzyme fatty acid synthase (FAS) was localised in the 6.3 x 10⁶g-min supernatant ('cytosolic' fraction) of homogenates of whole <i>Calanus</i> and was characteristic of a Type 1 multienzyme complex. The formation of fatty acids required the presence of acetyl-CoA, malonyl-CoA and was specific for NADPH as the reductant. The main products of the reaction were the saturated long-chain fatty acids stearic acid and palmitic acid, accounting for 54% and 25% of the total fatty acid product, respectively. The alcohol-forming enzyme, NADPH-fatty acyl-CoA oxidoreductase, from both <i>Calanus</i> the gourami roe was membrane-associated. Differential ultracentrifugation showed that a 6.3 x 10⁶g-min pellet (the 'particulate' fraction) was most active in the formation of long-chain fatty acohols. The specific activity of the enzyme in the gourami roe particulate fraction was over 100-fold higher than that in the <i>Calanus</i> particulate fraction. Both systems required the presence of an acyl-CoA generating system in the form of exogenous CoASH, ATP and Mg⁺⁺. The reaction was specific for NADPH as the reductant in the case of the gourami roe system. However, NADH substituted for NADPH in the <i>Calanus</i> system. Palmitic acid, in the presence of an acyl-CoA generating system, was the most effective substrate with the 18:0, 18:1 and 22:1 substrates giving progressively lower activities.