Notoplana acticola, a free-living polyclad flatworm, has a small but well-defined brain that controls behavior of peripherally based motoneurones. This is the most primitive brain currently being studied electrophysiologicaly, but little is known of its cytoarchitecture. Using a modified rapid Golgi method, we have investigated neurone configurations within this brain. Superficially the brain resembles those of other invertebrates, but its cells also possess many vertebrate features. There is a surprising diversity of cell types with complicated branching patterns. Multipolar neurones appear to be the most common type. A few typical invertebrate monopolar cells were also stained. Bipolar cells occur in the rind. Processes resembling dendritic spines were observed. Measurements of these indicate that they fall within the range found in other vertebrates and invertebrates. Small multipolar cells that could either be glial or interneurones were found scattered through the brain.