Niemann-Pick type C (NP-C) disease is a progressive neurodegenerative disorder characterized by the inappropriate accumulation of unesterified cholesterol in lysosomes . NP-C patients show various defects including hepatosplenomegaly, ataxia, dystonia and dementia. Most cases of NP-C are associated with inactivating mutations of the NPC1 gene , which encodes a protein implicated in the retrograde transport of sterols and other cargo from lysosomes . Furthermore, localization of the NPC1 protein to lysosomal/endosomal compartments is essential for proper transport . To create a model of NP-C disease in a simple, genetically tractable organism, we generated deletion mutations in two Caenorhabditis elegans homologs of the human NPC1 gene, designated npc-1 and npc-2. Animals mutant for npc-1 developed slowly, laid eggs prematurely, and were hypersensitive to cholesterol deprivation. Furthermore, npc-1; npc-2 double-mutant animals inappropriately formed dauer larvae under favorable growth conditions. These phenotypes in C. elegans provide a model system for both genetic and chemical suppressor screening that could identify promising drug targets and leads for NP-C disease.