Gram-negative bacteria have evolved numerous systems for the export of proteins across their dual-membrane envelopes. Three of these systems (types I, III and IV) secrete proteins across both membranes in a single energy-coupled step. Four systems (Sec, Tat, MscL and Holins) secrete only across the inner membrane, and four systems [the main terminal branch (MTB), fimbrial usher porin (FUP), autotransporter (AT) and two-partner secretion families (TPS)] secrete only across the outer membrane. We have examined the genome sequences of Pseudomonas aeruginosa PAO1 and Pseudomonas fluorescens Pf0-1 for these systems. All systems except type IV were found in P. aeruginosa, and all except types III and IV were found in P. fluorescens. The numbers of each such system were variable depending on the system and species examined. Biochemical and physiological functions were assigned to these systems when possible, and the structural constituents were analyzed. Available information regarding the mechanisms of transport and energy coupling as well as physiological functions is summarized. This report serves to identify and characterize protein secretion systems in two divergent pseudomonads, one an opportunistic human pathogen, the other a plant symbiont.