The presence of an endogenous circadian clock in the brain of an animal was first demonstrated in the cockroach Leucophaea maderae. However, the clock's cellular basis remained elusive until pigment-dispersing hormone-immunoreactive neurons, which express the clock genes period and timeless in Drosophila, were proposed as pacemaker candidates. In several insect species, pigment-dispersing hormone-immunoreactive neurons are closely associated with the accessory medulla, a small neuropil in the optic lobe, which was suggested to be a circadian clock neuropil. Here, we demonstrate that ectopic transplantation of adult accessory medulla into optic lobe-less cockroaches restores circadian locomotor activity rhythms in L. maderae. All histologically examined cockroaches that regained circadian activity regenerated pigment-dispersing hormone-immunoreactive fibres from the grafts to original targets in the protocerebrum. The data show that the accessory medulla is the circadian pacemaker controlling locomotor activity rhythms in the cockroach. Whether pigment-dispersing hormone-immunoreactive neurons are the only circadian pacemaker cells controlling locomotor activity rhythms remains to be examined.