1. Observations of earlier workers on the chromatic responses to local heating or cooling of the skin of Phoxinus have been confirmed. They have also been found to hold true for anaesthetized fish, in which finer control and observation are possible.
2. The results of a series of nerve-section experiments are held to exclude any possibility that the responses of the intact fish are mediated by a nervous reflex from thermoreceptors in the skin.
3. The responses of melanophores following section of the chromatic motor tracts, or when isolated from the body, are considered to be independent cellular responses.
4. Normal responses in the intact fish show clear temperature thresholds and are completely dependent upon continuation of the respiratory rhythm.
5. Possible physiological mechanisms for mediating these non-adaptive responses are discussed and a new hypothesis is put forward.