Lissencephaly-1, which is implicated in human diseases, is a protein required for normal cell division. In this study we examine the role of Lissencephaly-1 in the freshwater planarian Schmidtea mediterranea. The animals possess a high concentration of mitotic cells and provide an excellent opportunity to examine the role of proteins, such as Lissencephaly-1, in stem cell division. We show that Lissencephaly-1 is essential for mitotic progression in planarian stem cells and establish the system for future studies aimed at investigating signaling roles of Lissencephaly-1 implicated in stem cell regulation.