Localization of Paracentrotus lividus bep maternal mRNAs at the animal pole occurs by association with the cytoskeleton and involves a 54-kDa protein, called LP54, that is able to bind to the 3′ untranslated regions (UTRs) of bep mRNAs. We describe here the isolation and purification of this protein. Antibodies raised against purified LP54 allowed us to establish its localization in P. lividus eggs and embryos. This localization coincides with the mRNAs with which it is associated, that is, the animal pole in the egg, and, after fertilization, the regions derived from this part of the egg, and finally the oral ectoderm of the pluteus. Association with the cytoskeleton was shown by the copurification of LP54 in a microtubule preparation. Involvement in bep mRNA localization was demonstrated by microinjection of anti-LP54 antibodies in P. lividus eggs, which caused alteration of spatial distribution of bep3 mRNA.