1. The lateral line of Fundulus heteroclitus and Fundulus majalis is shown to react to tone at an intensity level of 20 dynes per sq. cm. at frequencies up to 200 or 300 cycles per second.
2. Evidence is given that the nerve can reproduce the stimulating tone frequency up to at least 180 cycles per second.
3. The response of the lateral line to the swimming movements of nearby fish is demonstrated.
4. Fundulus and several other species are shown to give strong spontaneous activity of the lateral line nerve.