Electrolytic microlesions were made at various sites in optic lobes of cockroaches Leucophaea maderae, and animals were assayed for circadian locomotor rhythms. Lesions placed in the lamina or medulla disrupted rhythmicity in only a few animals (3 out of 55). Lesions placed in or near the lobula produced a greater fraction of arrhythmic roaches (18 out of 45). Over half of these lesions either included or were centered in the cell body regions bordering the second optic chiasm and the lobula. The results suggest that the cell bodies and not the neuropile areas of the lobe are the crucial elements of the clock driving the cockroach's circadian activity rhythm.