We demonstrate that the avirulence gene avrRpm1, isolated from Pseudomonas syringae pv. maculicola strain Psm M2 via interaction with the Arabidopsis resistance gene RPM1, is also required for maximal virulence on this host. Two avrRpm1::Tn3-Spice marker-exchange mutants do not elicit a hypersensitive reaction on RPM1-containing Arabidopsis accessions Col-0 and Oy-0. Surprisingly, these mutants neither generate disease symptoms, nor grow in planta, after inoculation onto susceptible accessions Nd-0, Fe-1, and Mt-0. These deficiencies can be corrected in a merodiploid containing a wild-type avrRpm1 allele, and are not observed following gene-replacement with avrRpm1::Tn3-Spice alleles containing insertions just beyond the 3' terminus of the avirulence gene open reading frame. AvrRpm1 mRNA is expressed in low, but detectable amounts, in rich media. Induced accumulation of transcript is observed 3 h after shift to minimal media, and an avrRpm1::Tn3-Spice marker-exchanged reporter gene reaches maximal induction 30 min after shift. AvrRpm1 transcription starts 5 base-pairs 3' of the putative regulatory "hrp-box" cis-element found upstream of many P. syringae avr and hrp genes. Transcriptional induction of the marker-exchanged reporter gene in minimal media is enhanced by a carbon source. Induction in planta is the same in either resistant or susceptible Arabidopsis accessions, and is unaffected by the presence or absence of wild-type avrRpm1. As previously observed for many other P. syringae avr genes, transcriptional regulation of avrRpm1 in minimal media is dependent on hrpL and hrpS.