The zinc content of mosquitoes in various developmental stages was determined by spectrographic and microchemical analysis and use of zinc 65 and found to be five to ten times higher than other trace elements. Also the concentration of zinc in the mosquito was much greater than in other insects of different biological orders. Over 90 per cent of this element was localized in the Malpighian tubules at a concentration of 32 µg zinc per mg dry weight. The non-dialyzable form of zinc is loosely bound, for it was dissociated upon dialysis against ethylenediamine tetraacetate. The uptake of this trace element was correlated during larval growth with weight increase and required the presence of food particles. Furthermore, this uptake was different from that of cobalt which was not accumulated when offered as an inorganic salt or as vitamin B 12 . Zinc was not detectable in pooled egg masses, and once embodied by the larvae, was retained under fasting conditions and at a constant level throughout the pupal stage and as long as 14 days' adult life. Supplementation of the media with EDTA caused a marked inhibition of growth that could be completely reversed by the addition of zinc or zinc plus lead. The resultant pupae, however, contained less than 5 per cent of the normal amount of zinc and were essentially zinc-free; yet their rate of growth and gross appearance were normal.