Indole-3-acetic acid (IAA) is found in plants in both free and conjugated form. Among the conjugated IAA forms, there exists a unique class of proteins and peptides where IAA is a prosthetic group attached directly to the polypeptide structure. The first gene, for a bean protein with an IAA prosthetic group, was cloned and subsequently a related protein and several smaller peptides were identified in Arabidopsis. Although the importance of auxin in development and ripening of strawberry (Fragaria) fruit is well established, little was known about auxin metabolism in strawberry, or auxin functions in these important agricultural processes. We found that achenes and receptacles of Fragaria vesca cultivar Yellow Wonder contained conjugated indole-3-acetic acid (IAA) that was insoluble in organic solvents and yielded IAA after strong alkaline hydrolysis, suggesting that IAA was attached to proteins and this protein bound IAA appears to account for a significant fraction of the IAA in the fruit tissues. To investigate the strawberry IAA protein conjugate class further, a polyclonal antibody was produced to IAA-glycine attached to BSA. This antibody detects neutral indolic esters, monocarboxylic-amino acid IAA conjugates and IAA-proteins. Both achenes and receptacles of strawberry showed an immuno-detectable band at 76 kDa. 2-D polyacrylamide gel electrophoresis resulted in a broad cross-reactive spot, which was analyzed by LC-MS/MS following in-gel trypsin digestion. The strawberry IAA-protein was tentatively identified by peptide fragment analysis as an ATP-synthase.