The aphid Schizaphis graminum is dependent on Buchnera aphidicola, a prokaryotic endosymbiont. One of the functions of the endosymbiont is the synthesis of essential amino acids for the aphid host. Previously we have found that B. aphidicola has many of the genes that encode enzymes of amino acid biosynthesis. Using reverse transcriptase and the polymerase chain reaction, we have detected messenger RNA corresponding to genes involved in the synthesis of tryptophan, isoleucine, valine, leucine, and histidine.