The following results apply to Rhizopus nigricans Ehrenb. in particular, but experimental evidence is at hand that the general principles involved apply also to Botrytis cinerea Pers. and Penicillium no. 24 Thom. Most of the data have been derived from work with two layers of medium separated by a perforated mica plate. 1. The fungus shows a marked negative chemotropic reaction to a medium in which it has been growing for some time. 2. The hypothesis brought forward by Clark and Fulton, that this negative chemotropism is a reaction of the fungus toward its "staling substances," is conclusively substantiated by the following evidence: (a) the hyphae in one layer of medium turn away from another layer without spores if it contains their own staling products; (b) the hyphae always show a marked turning from the medium in which they are growing to any medium which is free of hyphae, no matter what the composition of that medium may be, unless the second medium contains the substances which exert a negative ch...