The anatomy of the neurons associated with the six fibers forming the dorsal longitudinal flight muscle (DLM) of Drosophila melanogaster has been investigated using a horseradis peroxidase lable. The two dorsal-most fibers are innervated by the same neuron whose cell body is in the dorsal, contralateral, mesothoracic region of the thoracic ganglion. The ventral-most four fibers are innervated by four neurons whose cell bodies are clustered in the ventral, ipsilateral, prothoracic region. The processes of all five of these cells ramify extensively in the dorsal part of the ipsilateral and contralateral mesothoracic neuromeres. A large interneuron has been discovered which is associated with the DLM and whose cell body is located contralaterally. Several neurons with small cell bodies on the ventral midline of the mesothoracic neuromere are also consistently labeled. A single fiber projects dorsally from a midline cell body, forms a Y-branch near the top of the ganglion and apparently sends an axon into each posterior dorsal mesothoracic nerve (PDMN) subsequently innervating the DLM.