AbstractWe have analyzed morphological changes affecting the lympho‐hemopoietic organs of the anadromous sea lamprey,Petromyzon marinusthroughout its life span. For this analysis, ammocoetes (2–4 years), premetamorphosing lampreys (nearly 5 years), metamorphosing lampreys, macrophtalmia stages (young adults) and parasitic adults (nearly 7 years) were used. The principal lympho‐hemopoietic organs in the ammocoete are typhlosole, larval opisthonephros and nephros‐associated adipose tissue. After metamorphosis, these organs degenerate, and their lympho‐hemopoietic tissue is replaced by dense connective tissue. The supraneural body and to a lesser degree, the definitive opisthonephros, are the main blood‐forming organs in adult lampreys. During larval life, lympho‐hemopoietic cells appear in the branchial area, associated with pharyngeal epithelium. These loci are not morphologically homologous to the thymus gland of jawed vertebrates. These results are discussed, with special emphasis on the importance of cell microenvironments in eluciding changes in different blood‐forming loci throughout the life cycle and their significance for the lamprey's