In most natural environments, association with a surface in a structure known as a biofilm is the prevailing microbial life-style. Escherichia coli has been a useful model for the study of biofilm formation. Here we analyzed the amounts of biofilm formed when E. coli was cultured in the presence of MOPS [3-(N-morpholino)propane sulfonatel. We used the "O'Toole and Kolter" method, which consisted of growing cells in PVC microtiter dishes and staining the formed biofilm with crystal violet. Our results showed that: 1) the addition of 100 mM MOPS to the rich Luria-Bertani (LB) medium increased the capacity of biofilm formation of several E. coli strains; and 2) the biofilm formed by cells growing in the presence of MOPS was more evident and well defined than that of cells cultured in LB-only medium. The improved ability of forming biofilms was maintained even for 60 h after removing MOPS from the medium, indicating that this improvement was due to a change in the metabolism of E. coli growing in the presence of MOPS or that, under these conditions, biofilm formation was favored.