AbstractThe pathway of the median giant fiber in the crayfish cephalic ganglion (brain) was reconstructed from serial sections. The median giant fiber in the para‐esophageal nerve runs anteriorly and enters the brain. After turning medially, it given of a branch which further divides into two branches. These branches run ventrally and outwards and finally enter the ipsilateral antennule nerve and ventrally. It decussates and makes contact with the contralateral median giant fiber near the midline. After decussating, the fiber again divides into two branches. One runs outwards and ventrally and finally enters the contralateral antennule nerve. The other runs anteriorly at first and then turns ventrally immediately behind the rostral cellular group. Near the ventral surface of the brain the fiber turns posteriorly and connects with a large soma 100–150 μ in diameter. In its course to the soma, the main fiber decreases in width to one‐eighth of the diameter of the axon in the para‐esophage