We identified a new Drosophila gene, peter pan (ppan), in a screen for larval growth-defective mutants. ppan mutant larvae do not grow and show minimal DNA replication but can survive until well after their heterozygotic siblings have pupariated. We cloned the ppan gene by P-element plasmid rescue. ppan belongs to a highly conserved gene family that includes Saccharomyces cerevisiae SSF1 and SSF2, as well as Schizosaccharomyces pombe, Arabidopsis, Caenorhabditis elegans, mouse, and human homologues. Deletion of both SSF1 and SSF2 in yeast is lethal, and depletion of the gene products causes cell division arrest. Mosaic analysis of ppan mutant clones in Drosophila imaginal disks and ovaries demonstrates that ppan is cell autonomous and required for normal mitotic growth but is not absolutely required for general biosynthesis or DNA replication. Overexpression of the wild-type gene causes cell death and disrupts the normal development of adult structures. The ppan gene family appears to have an essential and evolutionarily conserved role in cell growth.