1. About two and a half or three weeks after the attachment of Lernæopoda edwardsii to the brook-trout, the parasite is mature for fertilization. 2. The mature male is about one third as long as the mature female. The reproductive organs of the male are paired, and are located in the posterior region of the body, between the intestine and the body wall. They consist of a testis, a coiled vas deferens, and a spermatophore. 3. The female reproductive apparatus is also paired in character and is located within the abdomen, between the intestine and the body wall. The ovaries give rise to slender oviducts, which open at the lower extremity of the abdomen. Slightly above their terminations, the oviducts combine to form the spermatheca, which opens to the exterior through the two genital pores. Within each oviduct a spiral gland can be seen, which later develops into the cement gland. 4. In order to effect fertilization, the male makes circling movements while still attached to the host, thereby meeting a female. Then the male releases his hold on the host, attaches himself to the female, and moves down posteriorly in the vicinity of the genital pores. 5. The male bends his abdomen upward towards the female, and extrudes the spermatophores from the reproductive organs. These he manipulates with his second maxillæ and soon attaches them about the genital pores of the female. 6. The spermatozoa wander from the spermatophores into the spermatheca of the female and are stored until the eggs are ripe enough to undergo fertilization. 7. The eggs pass down the oviducts when mature, and are fertilized by the stored spermatozoa as they pass the spermatheca. The embryos pass into the external egg-sacs, where they develop into larvæ.