Oncospheres of Hymenolepis diminuta were hatched in vitro using extracts of adults or larvae of Dermestes vulpinus in Tyrode''s solution. Upon the addition of this extract, hatching occurs in 15 to 30 minutes after mechanical breakage of the egg shell. Oncosphere activity appears essential for successful hatching. The membranes enclosing the oncosphere of Hymenolepis diminuta are (a) the egg shell composed of roughly hexagonal blocks, (b) a thin membrane applied to the inside of the shell, (c) the vitelline membrane, (d) the oncosphere coat, and (e) a delicate membrane enclosing the oncosphere beneath the coat. During the process of hatching, mechanical friction is necessary for breaking of the egg shell and rupture of the underlying membrane. The vitelline membrane apparently is digested or dissolved by substances present in the host, while the oncosphere coat and membrane rupture only with prolonged oncosphere activity, whether or not the vitelline membrane has disappeared. Extracts of Dermestes vulpinus prepared with distilled water or normal saline solution instead of Tyrode''s do not bring about hatching. Hatching may be reduced or in-hibited by extracts prepared with Tyrode''s lacking certain components. Extracts of Tenebrio molitor in Tyrode''s solution, or Tyrode''s solution, alone, will not elicit hatching.