Provisional abstract and full-text PDF file correspond to the article as it appeared upon acceptance. Fully formatted PDF and final abstract versions will be made available soon.[Background] Dietary restriction (DR) results in increased longevity, reduced fecundity and reduced growth in many organisms. Though many studies have examined the effects of DR on longevity and fecundity, few have investigated the effects on growth.[Results] Here we use Caenorhabditis elegans to determine the mechanisms that regulate growth under DR. We show that rather than a reduction in cell number, decreased growth in wild type C. elegans under DR is correlated with lower levels of hypodermal polyploidization. We also show that mutants lacking wild type sensory ciliated neurons are small, exhibit hypo-polyploidization and more importantly, when grown under DR, reduce their levels of endoreduplication to a lesser extent than wild type, suggesting that these neurons are required for the regulation of hypodermal polyploidization in response to DR. Similarly, we also show that the cGMP-dependent protein kinase EGL-4 and the SMA/MAB signalling pathway regulate polyploidization under DR.[Conclusions] We show C. elegans is capable of actively responding to food levels to regulate adult ploidy. We suggest this response is dependent on the SMA/MAB signalling pathway.Our support came primarily from NERC (UK) and BBSRC. LST was also supported by Parkinson’s Diease Society (PDS). EL and AGS were also supported by programs I3P and Ramón y Cajal, respectively, and by Plan Nacional I+D (CGL2005-00307), all from the Ministry of Education and Science, Spain.Peer reviewed