The locus fushi tarazu appears to be involved in the establishment of the segmentation pattern of the Drosophila embryo. The cuticle of ftz mutant embryos is missing structures in alternating segments such that only half the normal number of segments are present. We have localized ftz+ transcripts in tissue sections of wild-type Drosophila embryos by in situ hybridization. Transcripts from the ftz+ gene were first detected during nuclear cleavage prior to cell formation. During the last two nuclear divisions ftz+ transcription becomes gradually restricted such that at the cellular blastoderm stage the ftz+ transcripts are localized in seven evenly spaced bands of cells. The size of each band is similar to the size of the segment primordia at the blastoderm. By the time segmentation becomes morphologically distinct ftz+ transcripts are no longer detected. These results suggest that the ftz+ gene plays a key role in the determination of the segmentation pattern in the embryo.