AbstractDuring the larval‐pupal transformation of the cecropia silkmoth, the concentration of magnesium in the fat body declines from about 40 to 20 mEq/I cell water, while intracellular trehalose drops to a similar extent. Among fullygrown larvae and early pharate pupae, the hemolymph trehalose level (which is close to the fat body trehalose level in these stages) is correlated with the concentration of magnesium in the fat body but not with that in the hemolymph.Injection into larvae, of sufficient magnesium chloride to double the magnesium content in the hemolymph, did not lead to elevated magnesium in the fat body.It is concluded that the concentration of magnesium in the fat body is regulated independently from that in the hemolymph. Since the trehalose phosphate synthetase is activated by magnesium, the concentration of magnesium in the fat body may play a significant role in regulating the rate of synthesis, and hence the steady state level, of trehalos