A cDNA clone encoding a phosphatidylinositol 3-kinase (PtdIns 3-kinase) has been characterized from Medicago truncatula, Mtpi3k, that is highly homologous to their counterparts from soybean (over 84%). The results suggest the presence of at least two genes coding PtdIns 3-kinases in M. truncatula. Mtpi3k transcript levels increased in nodules, compared with non-infected roots. Strikingly, Mtpi3k mRNA accumulated in young elongating stems at higher levels than that observed in other organs. Enhanced transcription of genes coding PtdIns 3-kinases might occur in tissues experiencing a high degree of vesicle trafficking and cell elongation.