The length of the uterus of Dicrocoelium dendriticum, Pleurogenoides medians and Fasciola hepatica at different stages of development is correlated with the corrected size of the body (parallel length x breadth). 2. The data plotted in a log-scale show for all three species a sigmoid curve with linear middle section. 3. The function y = ax + ac(xw - x)3 + b(y = log length of the uterus, x = log length of the body, xw = log length of the body at the turning point) fits very well to the data obtained from Dicrocoelium dendriticum. The constants a, ac and b were determined numerically. The function is valid also for Pleurogenoides medians and Fasciola hepatica but with other constants. 4. The linear middle section of the sigmoid curve may be interpreted by allometric growth. During this phase of development the uterus is tubelike. 5. During the earliest stages of development the uterus forms a solid cord of cells (three-dimensional growth) and grows slower than in a later stage after formation of a cell-free interior (two-dimensional growth). 6. The latest phases of development are characterized by a small increase in length of the uterus due to the three-dimensional dilatation by the produced eggs. 7. The computed function is considered as a modification of the law of allometric growth Y = B . Xa, respectively y = ax + b (log Y = y, log X = x).