An essential step in the Drosophila segmentation mechanism is the expression of the pair-rule gene hairy in a periodic pattern in the blastoderm. In this paper we describe four regulatory hairy mutations. Using in situ hybridization, we show that these mutations cause alterations in the normal pattern of hairy expression. The new patterns are partial versions of the wild-type pattern and indicate that there are regulatory sequences associated with hairy that respond to distinct cues in different parts of the blastoderm. This suggests that a major function of hairy is to decode a prepattern consisting of complex cues (probably generated by the coordinate and gap classes of segmentation genes) into a simple periodic pattern. We have located the mutations on the DNA map of the hairy gene. They identify a 5' region of approximately less than 20 kb necessary for this decoding function.