The majority of the genes for U1 RNA are organized in tandemly repeated units in the sea urchin. To assess the level of expression of these genes in the sea urchin Lytechinus variegatus, we measured the transcription of sequences 3' to the gene. The tandemly repeated U1 genes are expressed in morula and continue to be expressed at high rates until 2 hr after hatching, at which time the rate of expression of all the U1 genes and the tandemly repeated U1 genes declines sharply. By the gastrula stage the synthesis of total U1 RNA has declined by a factor of 8. The major tandemly repeated genes are inactive by this time, although other U1 genes remain active. The sequence of U1 RNA synthesized late in embryonic development differs from the sequence of U1 RNA encoded by the tandemly repeated set of U1 RNA genes, indicating that there must be other U1 RNA genes that are active late in embryonic development.