1. Males of Upogebia littoralis parasitized by Gyge branchialis show a marked increase in fat content from 1·04 to 1·6%.
2. Parasitized females have a fat content practically the same as that of ordinary non-ovigerous females, 1·2%.
3. Ovigerous females have a fat content considerably higher than that of parasitized males or non-ovigerous females, namely 2·2%.
4. The parasite Gyge has a relatively high fat content, 4·4%.
5. There is a significant diference in glycogen content between normal males and females.
6. There may be a slight increase in glycogen following parasitization.
7. The results are essentially in agreement with the theory of metabolic stimuli put forward by Geoffrey Smith.