Four large (Type I) interneurons with cell bodies in the anterior cluster of the brain and decussating axonal processes in an oesophageal connective initiate antennular withdrawal behavior in Callinectes sapidus. Recurrent neurites from the oesophageal process of each interneuron project into antennal and lateral antennular neuropils. These recurrent processes have varicosities on their terminal processes that are hypothesized to possess output synapses onto antennular withdrawal motoneurons. Secondary neurites of Type I interneurons in the medial antennular neuropil receive tertiary branches from antennal and lateral antennular neuropils. These secondary neurites in the medial antennular neuropil also receive short latency, excitatory input from mechanosensory afferent fibers from the antennule base. Intracellular depolarization of Type I interneurons produces antennular withdrawal ipsilateral to the descending axonal process. Intracellular recordings from identified withdrawal motoneurons during depolarization of Type I interneurons produces excitatory postsynaptic activity in these motoneurons that is of short latency. Hyperpolarization tests of Type I interneurons during stimulus-evoked fictive antennular withdrawal show that each is sufficient but not necessary for the production of withdrawal. Type I interneurons in Callinectes are morphologically and physiologically similar to interneurons previously described in Carcinus maenas.