Gypsy moth,Lymantria dispar(L.), adults and fourth instars reared from first instar on diet enriched with 3 g rubidium cloride (RbCl) per liter acquired the rubidium (Rb) tag without affecting their phenology or survival. The tag was retained in fourth instars at significant levels for 5 d after removal from the Rb-enriched diet. Loss of Rb after transfer from Rb-enriched diet to diet without Rb was described by a nonlinear regression model of the log of Rb concentration against days after transfer. AdultCarabus nemoralisMüller that consumed Rb-tagged fourth instars acquired the tag. Rb concentration in the beetles was positively correlated with the number of larvae eaten and negatively correlated with the number of days since feeding.