A cloned Lytechinus pictus cDNA has been identified, which includes seven direct repeats of a 228 bp sequence encoding ubiquitin and about 450 bp of 3' noncoding sequence. The deduced amino acid sequence is identical to that of ubiquitins of other animals (though repeats 3 and 5 each have single amino acid substitutions at different positions). Southern blot analysis revealed that the sea urchin genome contains a single copy of the polyubiquitin gene, and the number of 228 bp repeat units appears to vary from seven to ten among different alleles; no other ubiquitin coding sequences were detected. The size distribution of polyubiquitin mRNA is polymorphic among different individuals, probably corresponding to the differences in copy number of the repetitive coding sequence. The abundance of cytoplasmic polyubiquitin RNA is constant throughout embryogenesis and is similar in ectoderm, endoderm, and mesoderm cells. The constant prevalence of polyubiquitin mRNA apparently results from a balance between ontogenetic changes in its rate of synthesis and its stability in the presence of actinomycin D. Accumulation of polyubiquitin RNA was not heat shock-inducible during embryogenesis.