Schistosomula and adults of Schistosoma mansoni were grown from cercariae in cultures differing only in the treatment of the red blood cells fed to the organisms. "Pink ghosts," containing about 5% of the original hemoglobin, were produced by hemolysis in water; "white ghosts" with no detectable hemoglobin were made in 5 mM phosphate buffer, pH 8. Early growth and development were more rapid and vigorous, and pairs formed more readily when pink ghosts, rather than intact erythrocytes were fed. Schistosomula remained stunted and undeveloped when fed with white ghosts. Attempts at reconstitution of the latter by addition of hemoglobin, concentrated erythrocyte lysate, or pressure-liquefied pink ghosts did not restore growth-promoting activity. Pink ghost-fed worms, particularly paired males, attached to the dish bottom by their acetabulum and oral sucker and travelled by an active looping motion. Substrates of collagen or fibrin or a mammalian cell monolayer did not affect this behavior. Such attachment and locomotion are interpreted as instinctive migratory behavior of schistosomes.