During early development gene expression is controlled principally at the translational level. Oocytes of the surf clam Spisula solidissima contain large stockpiles of maternal mRNAs that are translationally dormant or masked until meiotic maturation. Activation of the oocyte by fertilization leads to translational activation of the abundant cyclin and ribonucleotide reductase mRNAs at a time when they undergo cytoplasmic polyadenylation. In vitro unmasking assays have defined U-rich regions located approximately centrally in the 3′ UTRs of these mRNAs as translational masking elements. A clam oocyte protein of 82 kDa, p82, which selectively binds the masking elements, has been proposed to act as a translational repressor. Importantly, mRNA-specific unmasking in vitro occurs in the absence of poly(A) extension. Here we show that clam p82 is related to Xenopus CPEB, an RNA-binding protein that interacts with the U-rich cytoplasmic polyadenylation elements (CPEs) of maternal mRNAs and promotes their polyadenylation. Cloned clam p82/CPEB shows extensive homology to Xenopus CPEB and related polypeptides from mouse, goldfish, Drosophila and Caenorhabditis elegans, particularly in their RNA-binding C-terminal halves. Two short N-terminal islands of sequence, of unknown function, are common to vertebrate CPEBs and clam p82. p82 undergoes rapid phosphorylation either directly or indirectly by cdc2 kinase after fertilization in meiotically maturing clam oocytes, prior to its degradation during the first cell cleavage. Phosphorylation precedes and, according to inhibitor studies, may be required for translational activation of maternal mRNA. These data suggest that clam p82 may be a functional homolog of Xenopus CPEB.