The phylogenetically ancient genomic islands of the abundant PAGI-2/pKLC102 family are prone to horizontal gene transfer amongst proteobacteria, and account for most genomic diversity in Pseudomonas aeruginosa. The mRNA expression levels of the sequenced PAGI-2 and pKLC102 islands were determined in P. aeruginosa clone C strains C and SG17M during exponential and stationary growth in Luria broth or Vogel-Bonner mineral medium. Of the 111 ORFs of PAGI-2, only one gene was significantly expressed at a level of more than 0.0001% of total RNA. The individual mRNA transcripts of the 103 pKLC102 ORFs, however, were present in the range of 0.001% to more than 1% in the bacterial RNA population, and amounted altogether to more than 10% of cellular RNA. Homologous genes were strongly transcribed from pKLC102, but not at all from PAGI-2 under the tested conditions. Thus PAGI-2, which was stably captured by its host chromosome, was transcriptionally silent, whereas the mRNA transcripts derived from the mobile and episomally replicating pKLC102 were constitutively more abundant in the cell than the mRNA pool transcribed from the core genome.