The proctodeum of the Drosophila embryo originates from the posterior end of the blastoderm and forms the hindgut. By enhancer-trap mutagenesis, using a P-element-lacZ vector, we identified a mutation that caused degeneration of the proctodeum during shortening of the germ band and named it aproctous (apro). Expression of the lacZ reporter gene, which was assumed to represent expression of the apro gene, began at the cellular blastoderm stage in a ring that encompassed about 10-15% of the egg's length (EL) and included the future proctodeum, anal pads, and posterior-most part of the visceral mesoderm. In later stages, strong expression of lacZ was detected in the developing hindgut and anal pads. Expression continued in the anal pads and epithelium of the hindgut of larvae; the epithelium of the hindgut of the adult fly also expressed lacZ. The spatial patterns of the expression of lacZ in various mutants suggested that the embryonic expression of apro was regulated predominantly by two gap genes, tailless (tll) and huckebein (hkb): tll is necessary for the activation of apro, while hkb suppressed the expression of apro in the region posterior to 10% EL. Cloning and sequencing of the apro cDNA revealed that apro was identical to the Trelated gene (Trg) that is a Drosophila homolog of the vertebrate Brachyury gene. apro appears to play a key role in the development of tissues derived from the proctodeum.