IN the cockroach Blatta orientalis there appears, in both laboratory experiments1 and in natural infestations2, a rhythm of activity under normal conditions of light and darkness taking the form of a burst of activity in the early part of the night, mainly before midnight, and a quiescent period in the later part of the night and during the day. This rhythm can be eliminated by keeping the insects under constant light, when activity becomes random; normal activity is, however, continued for at least three days under the new conditions. A similar rhythm occurs in Periplaneta americana 3.