The adult ovary of Drosophila is composed of approximately 20 parallel repetitive structures called ovarioles. At the anterior tip of each ovariole is a stack of 8-9 disc-shaped cells, called the terminal filament. Ovariole morphogenesis starts with the formation of the terminal filaments. Using two enhancer trap markers for terminal filament cells, we show that terminal filaments form in a progressive manner from medial to lateral across the ovary and that the number of terminal filament cells in a developing stack increases gradually. This process occurs during the second half of the third larval instar. One of these enhancer trap mutations, which is in the bric à brac gene, demonstrates that this gene is necessary for terminal filament formation and that a terminal filament cell cluster is required for ovariole morphogenesis to take place.