In locomotory systems, the central pattern generator and motoneuron output must be modulated in order to achieve variability in locomotory speed, particularly when speed changes are important components of different behavior acts. The swimming system of the pteropod mollusc Clione limacina is an excellent model system for investigating such modulation. In particular, a system of central serotonergic neurons has been shown to be intimately involved in regulating output of the locomotory pattern generator and motor system of Clione. There are approximately 27 pairs of serotonin-immunoreactive neurons in the central nervous system of Clione, with about 75% of these identified. The majority of these identified immunoreactive neurons are involved in various aspects of locomotory speed modulation. A symmetrical cluster of pedal serotonergic neurons serves to increase wing contractility without affecting wing-beat frequency or motoneuron activity. Two clusters of cerebral cells produce widespread responses that lead to an increase in pattern generator cycle frequency, recruitment of swim motoneurons, activation of the pedal serotonergic neurons and excitation of the heart excitor neuron. A pair of ventral cerebral neurons provides weak excitatory inputs to the swimming system, and strongly inhibits neurons of the competing whole-body withdrawal network. Overall, the serotonergic system in Clione is compartmentalized so that each subsystem (usually neuron cluster) can act independently or in concert to produce variability in locomotory speed.