Abstract Sea urchin embryo mRNAs of three distinct classes—histone mRNA, nonhistone mRNA containing poly(A), “[+A] mRNA,” and nonhistone mRNA lacking poly(A), “[−A] mRNA”—all contain blocked 5′ terminal sequences in which 7-methylguanosine is linked 5′-5′ via a triphosphate bridge to a 2′-0-methylated nucleotide. Only one general type of 5′ terminal structure, 7mGpppX mpYp, is present. Additional 2′-0-methylation in the Y residue has not been found either in early or late stage embryos. A substantial proportion of the polyribosomal mRNAs in all three classes contain this blocked structure. Whereas both classes of nonhistone mRNAs have internal base methylations, histone mRNAs lack such modifications.