The relative transcription of three unlinked actin genes of the sea urchin Strongylocentrotus purpuratus was measured in isolated nuclei, at several stages during embryonic development. Transcription of two cytoskeletal actin genes, CyI and CyIIIa, was first detected in 64-128 cell embryos. At the early stages, the CyIIIa gene is several-fold more active per embryo than CyI. The relative transcription of these two genes changes as development proceeds so that by the pluteus stage the CyI gene is at least twice as active per embryo as the CyIIIa gene. Both the time of initial detection of transcription of these two genes and the shift in their relative transcription during development correspond closely with the appearance and relative abundance in embryos of the mRNAs from these genes. Transcription of the muscle actin gene M was first detected in nuclei from pluteus stage embryos and thus also closely correlates with the first appearance of the muscle actin gene mRNA in embryos. The tight temporal coupling of the appearance in embryos of mRNA from these genes and the detection of their transcription in nuclei suggests that the regulation of their expression is in large part transcriptional. In addition to examining the transcription of these actin genes, we discovered that a member of an actively transcribed repeated sequence family is located upstream of the muscle actin-coding sequence. This sequence, which is present at least several hundred times within the genome and hybridizes strongly to RNA synthesized by RNA polymerase III at cleavage stages and to RNA synthesized in nuclei from pluteus stage embryos, shows little hybridization at blastula and gastrula stages.