1. Amœba proteus moves regularly and smoothly by alternate formation of pseudopods on the two sides of the organism. Locomotion in segments of Amœba with a nucleus is of the same general character. Movement in segments without a nucleus, however, is irregular, jerky, very much slower than that in nucleated parts, and the pseudopods are not ordinarily formed regularly or alternately. 2. In a horizontal beam of light, normal specimens direct their locomotion from the source of stimulation, i.e., they orient and are negative. Parts containing a nucleus respond in the same manner; those without a nucleus, however, do not orient. 3. The rate of locomotion varies greatly in different individuals. Large specimens move more rapidly than small specimens, the rate of locomotion bearing a fairly definite ratio to the size of the specimen. The rate of locomotion in nucleated segments bears essentially the same ratio to their size as it does in normal individuals. Segments without a nucleus show very little locomotion and this is always relatively very slow and irregular. 4. The size of the parts, the contractile vacuole, and the position which the parts occupied in the intact specimens before division seem to be in no way responsible for differences in the behavior of nucleated and enucleated parts of Amœba. 5. The only other known respect—aside from those mentioned—in which the two parts differ concerns the nucleus. Consequently the differences in the behavior of these parts are, in all probability, in some way related to the nucleus. 6. The regulatory influence of the nucleus on behavior in Amœba seems to be brought about by some sort of an influence upon the attachment of the organism to the substratum.