In the Drosophila compound eye the dorsal and ventral fields of eye units (ommatidia) meet along the dorsoventral midline, forming a line of mirror image symmetry called the equator(1), The molecular mechanism establishing the equator is not fully understood, but it involves the transcription factors＇ encoded by the Iroquois gene complex(3), The Iroquois genes are expressed in the dorsal half of the eye(2) and here we show that they regulate the expression of the secreted molecule Fringe. A boundary between fringe-expressing and fringe-non-expressing cells is essential, from the time of the second larval instar, for eye growth and formation of the equator. Boundaries of fringe expression determine where the transmembrane receptor Notch is activated(4,5). We find that Notch is activated at the dorsoventral midline, where it is required to promote growth and set up the axis of mirror symmetry. As boundaries of fringe expression and Notch activation are also important during Drosophila wing formation(6) and vertebrate somitogenesis(7-9), we suggest that these boundaries constitute a general mechanism that directs growth and patterning of large fields of cells.