Review of several aspects of the biology of Strongyloides westeri, especially on the life cycle, are presented. The main reason is because of the potential renewed importance of these parasites since the recently recognized increase in their prevalence in horse foals. Indications are that the first and main source of enteric infections in foals maturing to adults in their small intestines is from parasitic third stage larvae (L3) passing to them in milk of their dams. It appears that a lesser source of infection is from free-living L3 which, when they are ingested with food or penetrate the skin, can mature in foals. In conclusion, the source of enteric infections of adult S. westeri in foals is mainly from parasitic L3 that they ingest in milk of their dams.