The reproductive biology and behaviour of Theromyzon tessalatum is described from the commencement of copulation to the release of the young and the death of the parent. Copulation occurs several times over a10–20 day period with either the same or different partners. Eggs were laid6–14 days after copulation in usually four cocoons each containing approximately 100 eggs. The cocoons were brooded by the parents and the young, on development of the embryonic attachment organ and later by the posterior sucker, attached to the ventral surface of the parent's abdomen. The young detached from the parent either on contact with a suitable host for a blood meal or after approximately five months just prior to the parent's death.