The molecular basis for the recruitment of maternal mRNAs after fertilization is poorly understood, since there is little information available regarding the identity of such mRNAs. An assay based on reverse transcriptase and PCR was used to identify a maternal mRNA that undergoes a transient polyadenylation after fertilization; the length of the poly(A) tail increases from 40-80 adenosine (A) residues in the unfertilized egg to approximately 250 3 h after insemination and then decreases to about 180 around the time of pronucleus formation, that is 7 h after insemination. The DNA sequence of this cDNA, which encodes the polyadenylation signal AAUAA, contains a uridine-rich sequence that may serve as the cytoplasmic polyadenylation element observed in other maternal mRNAs that are recruited after fertilization.