Abstract Larvae of Drosophila contain a glutamate dehydrogenase which is localized mainly in the mitochondria. The enzyme operates with either NAD or NADP as coenzyme, and is unusual in that NAD is more readilyutilized in one reaction whereas NADPH is more efficient in the other. Like other animal glutamate dehydrogenases, the Drosophila enzyme is sensitive to a variety of modifying agents. Amongst the purine nucleotides, 5-AMP, cyclic 3′-5′- and 2′-3′-AMP, and, most especially, ADP activate the enzyme, whereas GTP, GDP, and to a lesser extent ATP, inhibit. Diethylstilbestrol and zinc are inhibitory, and EDTA causes slight stimulation. The pH optimum for the enzyme is dependent on coenzyme, the presence of ADP, the concentration of ammonia, and the presence of phosphate. Michaelis constants are affected by ADP. The similarities and differences between the Drosophila glutamate dehydrogenase and those from other animals are outlined, and the role of the enzyme in amino acid metabolism is discussed. It is concluded that insect glutamate dehydrogenases may have a primary role in regulating the use of amino acids for energy production.