The expression of interspersed repetitive sequences in the RNA of mature sea urchin oocytes was investigated. 3H-DNA tracers representing short interspersed repetitive sequences a few hundred nucleotides long, and long repetitive sequences approximately 2000 nucleotides long, were prepared from genomic DNA of the sea urchin, Strongylocentrotus purpuratus. These tracers were reacted with excess RNA from the mature oocyte. About 80% of the reactable short repeat tracer and 35% of the long repeat tracer hybridized. Thus most of the repetitive sequence families in the short repeat tracer are represented in oocyte RNA, and transcripts complementary to both strands of many repeat sequences are present. The kinetics of the reaction show that some transcripts are highly prevalent (greater than 10(5) copies per oocyte), while others are rare (approximately 10(3) copies per oocyte). Nine cloned repetitive sequences were labeled, strand-separated and reacted with the oocyte RNA. Transcripts of both strands of all nine repeats were found in the RNA. The prevalence of transcripts of the cloned repeat families varied from approximately 3000 to 100,000 copies per oocyte. Studies with both cloned and genomic tracers show that transcript prevalence is independent of the genomic reiteration frequency of the transcribed repetitive sequences. Most of the families represented by prevalent transcripts have fewer than 200 copies per haploid genome. The RNA molecules with which the cloned repeats react are at least 1000-2000 nucleotides in length. Other experiments show that a majority of the members of repeat families represented by prevalent transcripts in the oocyte RNA are interspersed among single-copy sequence elements in the genome.