Odd-numbered fatty acids are present in natural sources of animal or vegetable origin. Their concentration is generally not higher than 5% of the total fatty acids. In some cases, however, like in the mullet or Euglena gracilis, the value reaches 20% of the total fatty acids. Higher concentrations, up to 80%, have been found in some hydrocarbon-grown microorganisms. In the animal organisms metabolic pathways for the synthesis or degradation of the odd-numbered fatty acids are present, but the adaptive capacity and the regulation of the different pathways are not known. The consequences of feeding animals or human beings on these unusual fatty acids in large amount are uncertain, because the composition of the fat depots, of the lipid structures and the circulating lipids can be affected, possibily influencing the membrane functions and, particularly the nervous system functions. The literature sighted shows the scarcity of specific studies in this field and indicates the need for further and more detailed researches.