To better understand the interaction between bacteria and surfaces, we studied the irreversible attachment of Pseudomonas aeruginosa to a common surfacing material. When brought into contact with the steel, cells began to attach in less than 1 min and the number adhering increased with time. An important physiological variable in attachment was cell motility since adherence decreased at least 90% when flagella were removed by blending. This treatment was shown to be effective because it caused motility loss and not because it removed a structure necessary for adherence. Cell viability was less important since adherence decreased only 50% when the number of viable cells was reduced 4.7 logs by heating or formaldehyde treatment. Significant environmental variables included turbulence and ionic strength. Attachment of motile cells was reduced 90% by agitation, although agitation had little effect on adherence of nonmotile cells. Both motile and nonmotile cells adhered poorly in distilled water with attachment increasing as CaCl2 or NaCl concentration increased to 10 mM. At 100 mM, attachment decreased. Viable cells, both motile and nonmotile, adhered best at a pH of 7 to 8, whereas nonviable cells attached most rapidly at a low pH.