With 5 plates and 5 figures in the text) Theromyzon tessulatum were reared individually from hatching until they attained sexual maturity. They were fed at different seasons and cultured at one of two temperature regimes. A series of weight measurements enabled growth curves to be constructed. Observations were also made on the digestive processes. In keeping with at least one other sanguinuous glossiphoniid, a restricted and constant number of blood meals precede reproduction and the hungry and sated stages show contrasting and distinctive behaviour. A notable feature is the great increase in weight after feeding resulting from the absorption of water and its incorporation into the body tissues. It probably enters through the body surface. The role of microorganisms within epithelial cells of certain crop caecae, was also considered.