Accelerated weight gain caused by infection with spargana of Spirometra mansonoides occurs in male mice as well as females, and in older mice approximating 40 g starting weight, as well as in younger animals. Below a starting weight of 10 g the effect is greatly reduced and delayed if seven spargana per mouse are used. Deer mice, Peromyscus, show the same response. The oriental worm, S. ranarum, has an even greater stimulating effect than mansondoides, but is more antigenic, becoming encapsulated in the first 2 months, after which the effect is lost. The weight gain effect is additive, increasing with the number of spargana, but reaches a plateau at about 12 worms per 20-g mouse, after which either no further gain occurs, or gains are offset by increased mortality of the mice. Hamsters show an even more pronounced effect. Both males and females respond. In this case the infected animals grow at a very similar accelerated rate, while the controls show wide variation. True growth as well as obesity is involved, since infected hamsters are not only heavier but larger as shown by inspection and by X-ray. Small numbers of spargana appear to have no effect on longevity; a group of the original experiments allowed to die off naturally snowed similar mortality rates for both experimental and control mice. Nor is there any effect on reproduction. Infected mice and hamsters breed as well as controls if given the opportunity.