1. Neuronal mapping studies were conducted in brain and eyestalks of the crab, Cancer antennarius, directed to finding the unit(s) that control neurosecretion of molt-inhibiting hormone (MIH). Control is known to be serotonergic. 2. Nine clusters of neuronal somata were identified in the brain by direct staining with Toluidine Blue. Eight clusters are bilaterally arranged and the ninth occurs in the protocerebrum. 3. Separately, nine groups of cell bodies on each side of the brain were demonstrable by nickel-lysine backfilling of an optic peduncle. In eight of the groups, most of a total of about 250 cells on each side have not previously been identified in decapod crustaceans. 4. Backfilling of an optic peduncle also localized two identifiable axon tracts that filled into the contralateral eyestalk through the brain. Both are associated with filled cells in the eyestalk medulla terminalis, and one projects branches into the m. interna. Neither showed anatomical connection with the neurosecretory X-organ, sinus gland complex. 5. Except for cells in the protocerebrum, none of the neural units that backfilled appear to correspond with cells previously shown to be serotonergic. Together, these observations indicate that serotonergic control of MIH secretion resides entirely within the ipsilateral eyestalk.