The fushi tarazu (ftz) gene is essential for segmentation of the Drosophila embryo. This requirement is reflected at the cellular blastoderm stage of embryogenesis by seven transverse stripes of ftz expression. These stripes correspond to the missing segments of ftz mutant embryos. We describe here novel intermediate patterns of ftz protein expression which were detected in younger embryos by using anti-ftz antibodies and a sensitive fluorescence/immunoperoxidase technique ('filtered fluorescence imaging', FFI). Striped patterns of ftz protein evolved continuously, and the different stripes appeared in an ordered sequence, involving both anterior-posterior (A/P) and dorsal-ventral (D/V) progressions. Comparison of these patterns of ftz protein with those of ftz RNA suggests that these novel aspects of the patterning process involve post-transcriptional regulation in addition to the transcriptional control known to be involved in expression of this gene.