runt is one of the genes required for establishment of the segmented body pattern of the Drosophila embryo. We have isolated DNA sequences containing this gene using P-element transposon tagging. Southern blot analyses of six different DNA rearrangements that are associated with runt mutations revealed a minimal region of 8.5-kb of DNA that was important for function. In germ line transformation experiments, a 14.5-kb segment of DNA that spanned this minimal region provided significant, although not full, levels of runt activity. The runt gene encoded a 2.6-kb poly(A)+ RNA that underwent a series of dynamic changes in its spatial and temporal patterns of accumulation during embryogenesis. The runt RNA was most abundant at the blastoderm stage when it showed the seven stripes of expression characteristic of other Drosophila pair-rule genes.