Free amino acids were analyzed in healthy and rust-infected bean leaves incubated for 180–200 min in light, with 14CO2. In rusted leaves the radioactivity of the pool diminishes gradually to 42% in the flaking stage and the beginning of sporulation and then gradually increases in the following days to values of 197% after the secondary sporulation 11 days from infection. An increase in percentage of total labelling of aspartic acid, glutamic acid, and asparagine was noted, except just before sporulation, but above all, a marked and gradual increase in the labelling of glutamine was observed. This amide is probably assimilated to a greater degree by the fungus for the synthesis of its cell wall glucosamine. Diminution in the labelling of glycine and even more in the labelling of serine would instead be ascribed to the decrease in the photorespiratory activity.