Transcripts from the Xenopus gene GS17 are absent from oocytes and first appear at the midblastula transition (MBT), about 8 hr after fertilization. Injection experiments with deletions of the GS17 gene and GS17-beta-globin hybrid genes show that a relatively short sequence of 74 base pairs is sufficient to activate, in cis, transcription at the MBT. This 74-base sequence is located about 700 bases upstream of the GS17 promoter and has the properties of an enhancer element.