The Drosophila Btk29A tyrosine kinase is suggested to be involved in diverse processes, although its target proteins are unknown. In the present study, we investigated substrates of Btk29A tyrosine kinase by expressing a catalytically activated form of Btk29A-P1 (Btk-EG) in Drosophila compound eyes. Expression in eye disks led to the development of the rough-eye phenotype and increased tyrosine phosphorylation of a 65-kDa protein. Partial amino acid sequence analysis of this protein showed that it was phosphoglucomutase. Phosphoglucomutase activity in heads from Btk-EG-expressing flies was higher than that in controls, suggesting that the levels of tyrosine phosphorylation and activity of the enzyme are associated with Btk29A tyrosine kinase activity.