We have compared the outer membrane protein (OMP) pattern of sessile Pseudomonas aeruginosa cells with that of its stationary-phase free-floating counterparts. Two-dimensional electrophoretic gels were compared using computing scanning densitometry. The level of 103 OMPs (i.e. about 50% of the spots that fulfilled our statistical selection threshold) was significantly and reproducibly different in biofilm organisms as compared with their planktonic counterparts. Some OMPs that specifically either accumulated or decreased were identified by mass spectrometry and the role of these proteins in the biofilm phenotype is discussed.