The sea urchin early histone H1 gene is expressed only during a very short interval in the life cycle of the organism. In addition to a pool of stored maternal transcripts, the gene is active from the 16-cell stage to the early blastula stage of development, at which time the gene is transcriptionally repressed. We have defined the minimal sequences required for the proper temporal expression of this gene during early embryogenesis. These sequences from -65 to +39 include the TATA element, a conserved sequence CCCACGTACGCAA (part of USE 0) just upstream of the TATA element, an Inr element at the transcription start site, and an internal sequence in the leader region of the H1 transcript. Surprisingly, we could find no role for two highly conserved elements, a GC box (USE I) and an H1-specific element (USE II), that are important in the expression of vertebrate H1 genes. The internal element functions as a positive regulator when three copies are ectopically placed upstream of an early histone H3 TATA box. A temporal regulator is localized within the USE 0 sequence CCCACGTACGCAA, and it apparently plays a role in activation and probably repression of the gene.