The role of phosphorus (P) in numerous important biological structures, coupled with the observation that P-content of many insect foods is disproportionately low, suggests that P may be a critical nutrient for growing insects - however, the few studies examining the effects of dietary P on insect performance have generally found only weak relationships. This mismatch may be reconciled by understanding the physiological mechanisms by which insects handle P. Here we describe P processing by larvae of Manduca sexta. When given un-manipulated leaves of a common host plant, Datura wrightii, fifth-instar larvae retained about 85% of P consumed; when given P-enriched leaves larvae retained only 25% of P consumed. Analysis of gut concentrations of P at four sites along the digestive tract, and in leaves and feces, indicates that the rectum is the primary site of P transport between the gut and body and that differences in P retention may be accounted for by differential rates of rectal P transport. Larvae given P-enriched leaves also showed an eightfold increase in the concentration of P in the hemolymph, primarily as alpha-glycerophosphate - but only a 12% increase in the concentration of P in body tissues, suggesting that hemolymph plays a central role in storage and buffering of P.