Recently, it was reported that a deficit in the mouse stearoyl-CoA desaturase 1 gene decreases biosynthesis and accumulation of fatty acid and revitalizes the beta-oxidation of fatty acid. To examine the physiological role of fatty acid desaturase (FAT) and elongase (ELO)-gene transduction in ontogeny, fatty acid accumulation and individual lifespan, we performed bacteria-mediated RNA interference (RNAi) in the nematode Caenorhabditis elegans. Suppression of the expression of FAT-2 gene mRNA caused a drastic decrease in the amount of body fat and defects in egg-hatching. The amount of body fat was markedly decreased, and body size reduced, by down regulation of FAT-6 and FAT-7, whereas lifespan was drastically reduced. RNAi of the FAT-2 gene caused a remarkable increase of the beta-oxidation-related gene expression and the DAF-16 transcriptional activity, whereas, ELO-2 RNAi caused a remarkable decrease in fatty acid biosynthesis-related gene expression. Additionally, RNAi of FAT-6 decreased the mRNA levels of the genes involved in fatty acid synthesis, and FAT-7 RNAi increased the mRNA levels of beta-oxidation system genes. These results indicated that the elongation and desaturation of fatty acids are integral to various phenomena such as ontogeny and lifespan and play important roles in fatty acid accumulation and consumption.