In Ascaris suum chitin is formed in the zygote immediately after oocyte fertilization, and its synthesis is completed in the eggs from the distal half of the uterus. Incorporation of radiocarbon [14C] glucose into chitin of the eggshell was 40-fold higher than incorporation of [14C] glucosamine. The same rank order also holds for the incorporation of label from these isotopes into the glycogen of the ovaries. A large part of the radiolabel was incorporated first into oocyte glycogen and only after fertilization was it incorporated into eggshell chitin. Actinomycin D inhibited chitin synthesis in the eggs from the distal half of the uterus and it significantly reduced incorporation of radiocarbon from glucose into chitin.