The NADP(+)-specific glutamate dehydrogenase in Escherichia coli K12 has been shown to be phosphorylated in vivo when the cells are grown in a low-phosphate minimal salts medium containing P-32 inorganic phosphate. The amount of radioactivity incorporated into the enzyme is different depending on the growth phase of the culture, with the highest level of P-32 incorporation occurring during the mid-exponential growth phase. Previously reported studies have demonstrated that the enzyme is also phosphorylated in vitro in an ATP-dependent reaction.