During animal development, blast cell lineages are generated by repeated divisions of a mother cell into a series of daughter cells, often with a specific series of distinct fates. Nanos is a translational regulator that is involved in germline development in diverse animals and also involved in somatic patterning in insects. Recently, Nanos was found to be required for maintenance of stem cell divisions in the Drosophila germline. We have found that in the mollusk Ilyanassa, Nanos messenger RNA and protein are specifically localized in the mesendodermal blast cell lineage derived from the strongly conserved 4d cell. Nanos activity is required for differentiation of multiple tissues that are derived from the 4d cell, showing that IoNanos is required for somatic development in this embryo. At the cellular level, we show that IoNanos activity is required for the highly stereotyped cleavage pattern of the 4d lineage, the proliferative capacity of the blast cells, and the marked asymmetry of the blast cell divisions. These results suggest that IoNanos is involved in regulating blast cell behaviors in the 4d lineage.