The bendless (ben) mutation of Drosophila alters synaptic connectivity between a subset of CNS neurons. Here, we show that ben also causes morphological abnormalities within the visual system, suggesting that ben functions in a number of different developmental processes. We show that the ben gene encodes a protein which is closely related to ubiquitin-conjugating enzymes and that a missense mutation in the highly conserved active site region is associated with the ben mutation. High levels of ben expression are restricted to the nervous system during development. These results suggest a role for ubiquitin-mediated protein modification in nervous system development, including, but not exclusive to, the regulation of synaptic connectivity.