Optical recording techniques using voltage-sensitive dyes were used to examine the initiation and propagation of action potentials within neurons of the supraesophageal ganglion of the giant barnacle, Balanus nubilus. When a neuron was stimulated with current injection into the soma, the site of spike initiation, determined as the location with the earliest time-to-peak, was usually in the axon, 100 to 200 micron from the soma. The soma spike was broader and often later, suggesting that the cell body was less excitable than the axon. The action potential was narrowest at the site of initiation and then widened before propagating uniformly down the axon. In most cases, somatically stimulated action potentials and electrotonic pulses propagated into the dendrites with little delay or change of shape, indicating that the electrotonic length of these processes was great. Several different kinds of experiments indicated that some dendrites of these cells are excitable. These included the observations that (a) spikes could be made to initiate earlier in the dendrites than in the axonal region to which they were connected, and (b) action potentials sometimes decremented less than subthreshold pulses along dendritic processes. However, in other cases a decline in amplitude and a widening of the action potential demonstrated passive propagation into the dendrites, suggesting that not all dendrites are equally excitable.