Abstract 1. 1. Dyakia is the world's only known bioluminescent terrestrial snail. 2. 2. Since Dyakia's bioluminescence may be involved in social communication, a study of the visual system was carried out. 3. 3. Extracellular recordings from the optic nerve revealed an on-response. 4. 4. Effects of varying light intensity, duration and wavelength were viewed. 5. 5. The on-response of Dyakia may be considerably more complex than that of other pulmonate snails.