Abstract: Investigated the responses of amphipods to light. A series of experiments were conducted to determine the mechanism of the phototactic response, and the cause of reversal in the sense of phototaxis. Varying numbers of Ss were tested on reaction time to light and heat. The more terrestrial forms among the amphipoda were more positive. Concluded that the conditions which brought about the positive phototaxis were the same, as prevailed in the environment of the more terrestrial amphipoda. Heat and dryness favored positive reactions, while cold, moisture and silence favored negative reactions. The small specimen of O. traskiana had shorter periods of negative reactions than the more sluggish specimen, which could be made positive to the light by enforced activity, dryness and heat, resulting in metabolic processes through which chemical responses necessary for a response took place. The modification of the response helped in adaptation. (PsycINFO Database Record (c) 2016 APA, all rights reserved)