The Hms(+) phenotype of Yersinia pestis promotes the binding of haemin or Congo red (CR) to the cell surface at temperatures below 34 degrees C. We previously demonstrated that temperature regulation of the Hms(+) phenotype is not controlled at the level of transcription. Instead, HmsH, HmsR and HmsT are degraded upon a temperature shift from 26 degrees C to 37 degrees C. We used random transposon mutagenesis to identify new genes involved in the temperature-regulated expression of the Hms phenotype. One of these genes, which we designated hmsP, encodes a putative phosphodiesterase with a conserved EAL motif. Mutations in hmsP caused formation of red colonies on CR plates at 26 degrees C and 37 degrees C. Strains complemented with hmsP(+) on a plasmid form white colonies at both temperatures. We used a crystal violet assay and confocal laser scanning microscopy to demonstrate Hms-dependent biofilm formation by Y. pestis cells. Y. pestis Hms(+) strains grown at 26 degrees C but not at 37 degrees C form a biofilm on borosilicate glass surfaces. Strains that either overexpress HmsT (a GGDEF domain protein) or have a mutation in hmsP produced an extremely thick biofilm. Alanine substitutions for each of the GGEE residues (amino acids 296-299) of HmsT as well as the E506 and L508 residues of HmsP caused a loss of function. We propose that HmsT and HmsP together control the amount of biofilm produced in Y. pestis. Degradation of HmsT at 37 degrees C may be a critical factor in controlling the temperature-dependent expression of the Hms biofilm.