It has been shown (1, 2) that the capacity for building up visual conditioned responses is common to many types of fish when the unconditioned stimulus used as the basis for the “conditioning” is food. Preliminary work at Plymouth, using the fish Blennius gattorugine, showed that was possible that a knowledge of wave-length discrimination in that species might be acquired by the now familiar method of “conditioning” to an electric shock. This work was continued when I came to Cullercoats, and six specimens were used which were sent from Plymouth in 1929. All of these died within a short space of time during the succeeding winter months, owing to the intense cold. It was therefore necessary to restrict the investigation to Blennius pholis L., which is both plentiful and hardy at Cullercoats. Some progress had already been made and in Part II (2) it was intimated that an analysis would be made of the capacity for wavelength discrimination in Blennius pholis using an electric shock as the unconditioned stimulus. The present paper gives the results of that enquiry as far as it was taken.