In an attempt to interpret the distribution of Ascaridoidea in Mammals and Birds, a scheme is proposed, based on the concept of geological periods of evolutionary expansion for parasitic lines. It is assumed that the evolutionary expansion period of Ascaridoidea in Mammals occurred after the diversification of primitive mammals; this would explain the absence of ascarids in these groups. Periods of evolution appear to coincide with the diversification of Carnivora and Sirenia, the first to become invaded; Artiodactyla and Equidae became invaded later, at the time of their diversification. The other cases of parasitism of Mammals by Ascarids are thought to be "captures".