Synaptic connections were found between two groups of neurons in the CNS of Aplysia, the cerebral ganglion A and B cluster neurons which are involved in the control of pedal and parapodial movements and neurons in the pleural ganglion which has been shown to modulate locomotion. The A and B neurons made synaptic connections in both the cerebral and pleural ganglia. Pleural neurons had synaptic connections among themselves and with A and B neurons. The A neurons made excitatory monosynaptic connections with the B neurons and a minimum of 6 pleural neurons including the left giant cell (LGC). All of the A neuron synapses found were excitatory. The B neurons received excitatory synaptic input from two other groups of neurons in the cerebral ganglion and both excitatory and inhibitory input from pleural neurons. The latter were identified on the basis of their synaptic connections with the LGC and A neurons. The B neurons and LGC had several common synaptic inputs. The A neurons received monosynaptic input from only 2 pleural neurons. Complex synaptic circuits between A and B neurons and pleural neurons were found. These included recurrent inhibition of B neurons by A neurons via a pleural interneuron, feedforward summation of A neuron synaptic input to the LGC, and reciprocal excitatory synaptic connections between B and the pleural neurons. The activity of the B neurons was modulated by direct inhibitory and excitatory synaptic connections from pleural neurons. The A neurons were modulated primarily by a polysynaptic pathway through the B neurons. The modulation of cerebral A and B neurons by pleural neurons is consistent with behavioral results obtained studying locomotion.