1. An apparatus is described whereby it is possible to measure the propulsive forces generated during the locomotion of small terrestrial animals.
2. It is shown that the propulsive thrust of the circular muscles and the tractive force of the longitudinal muscles of an earthworm are approximately equal to each other and vary from 2 to 8 g. according to the size and activity of the worm.
3. The total frictional resistance overcome by a worm 25 cm. in length (when fully extended), moving at an average velocity of 0.4 cm. per sec., is approximately 8 g.