Cleavage and gastrulation initiation in Caenorhabditis elegans embryos are characterized by an invariant temporal and spatial pattern of cell divisions and cell movements. Although bulk embryonic transcription does not begin until gastrulation onset, some transcription can be detected as early as the 4-cell stage. To determine whether any early transcripts are required for normal cleavage-stage patterning, we blocked transcription in embryos by injecting hermaphrodite parental gonads with RNA antisense to the ama-1 gene, which encodes the large subunit of RNA polymerase II. This treatment prevented the expression of a reporter gene driven by an early embryonic promoter but did not detectably perturb the maternally controlled segregation of the germ line P granules or the pattern of cell division through the first four cleavages. In the fifth cell cycle, however, the two endodermal precursor (E) cells divided early and abnormally and failed to initiate gastrulation. The embryos arrested between the sixth and seventh cell cycles with less than 100 cells. These results indicate that embryonically transcribed gene products are required for gastrulation initiation. They also demonstrate the efficacy of a method for blocking embryonic transcription that may be useful in other organisms.