The messenger RNA (mRNA) differential display technique was applied to the identification and isolation of genes whose transcription was altered in leaves of Prunus armeniaca infected by European stone fruit yellows (ESFY) phytoplasma belonging to ribosomal subgroup 16SrX-B. Four genes whose steady-state levels of expression significantly changed in response to phytoplasma infection were isolated and identified. The results obtained show that two group of genes are affected by phytoplasma infection in apricot leaves. The first group comprises genes that are up-regulated by phytoplasma presence: in particular, a gene encoding the heat-shock protein HSP-70, a gene encoding a metallothionein (MT) and another homologous to the EST 673 cDNA clone of P. armeniaca, whose function was unknown. The other gene identified in our analysis is down-regulated by phytoplasma presence. It encodes a protein having homology to an amino acid transporter of Arabidopsis thaliana. Our findings demonstrate the usefulness of mRNA differential display approach for the detection of plant metabolic pathways affected by phytoplasma infection.