Most benthic marine invertebrates have long-lived planktonic ciliated larvae that must feed and grow to reach metamorphosis. Because ciliated cells in animals are unable to divide it is of considerable interest how ciliated larvae are able to grow. To understand how ciliated larvae grow I compared cell proliferation patterns in several species with planktotrophic larvae from five different phyla (Nemertea, Mollusca, Phoronida, Echinodermata, and Annelida). Cell proliferation events were detected using anti-phosphohistone antibody labeling, BrdU assays, and confocal microscopy. Studied larvae included some with monociliated epithelia (pluteus, bipinnaria, actinotroch, and mitraria) and others with multiciliated epithelia (metatrochophore, pilidium, and veliger). Dividing cells were detected in all studied larvae, but the pattern of dividing cells varied among types and correlated with the kind of epithelium (mono- vs. multiciliated) and phylogeny (e.g. protostome vs. deuterostome). Running z-projection movies of the actinotroch, mitraria, veliger and pilidium are included as supplemental files.