Abstract The thoracic ganglion of the blowfly, Sarcophaga bullata (Parker), was studied by electronmicroscopy. At least three types of neurons may be distinguished in the cortex of the ganglion: Type 1 neuron, about 15 μ dia.; Type 2 neuron, about 5 μ dia.; and giant nerve cells about 35 μ dia. These cells are surrounded by glial processes which penetrate into invaginations of the cytoplasm of Type 1 neurons and giant cells. The Type 2 neuron appears to be bipolar whereas the other cells seem to be unipolar. The cytological features of these cell types are discussed at the fine structural level. The nerve fibres of the perikaryon extend into the neuropile where they form a complex system of profiles. Glial processes also extend into the neuropile where they become interpolated among many of the nerve fibres. Three types of nerve fibre contacts are distinguished: end knob contacts, longitudinal contacts, and cross contacts. neurofilaments, mitochondria, synaptic vesicles, neurosecretory droplets, and agranular endoplasmic reticulum are found in the nerve fibres. Synaptic sites between nerve fibres may be distinguished by the presence of synaptic vesicles at synaptic foci in a nerve fibre as it makes naked contact with another fibre. All synaptic sites in the central nervous system are limited to the neuropile.