The morphology of the first abdominal ganglion of Limulus polyphemus (L.) was examined by reconstructions from stained serial sections. This ganglion, one of a chain of eight which form the ventral cord, is bilaterally symmetrical and shows the typical invertebrate division into two zones, cellular rind and fibrous core. The core of the ganglion comprises two major longitudinal tracts which are continuous with the connectives between the ganglia and four areas of regional neuropile which occur at the roots of the nerves developed from the ganglion. Minor tracts and neuropile are present. The rind is divided into several cell groups: U-shaped lobes around the nerve roots, a lateral lobe between the nerve roots and two cell layers along the ventral aspect. Six cell types are recognized: encapsulated unipolar cells, secretory unipolar cells (two types), encapsulated bipolar cells, “neuroglandular” bipolar cells and small non-encapsulated cells.