Abstract: Structural Fe in ferruginous smectite (sample SWa-1, Source Clays Repository of the Clay Minerals Society) was reduced by a mixture of five Pseudomonas species of bacteria in a defined Fe-free medium to determine the effect of microbial reduction on clay swelling. Iron(II), total Fe, and gravimetric water content (mw/mc) were determined in clay gels equilibrated at applied pressures of 0.1, 0.3, and 0.5 MPa. The water content of microbially reduced SWa-1 decreased at all three applied pressures as the Fe(II) content approached about 0.8 mmol Fe(II)/g-clay. As Fe(II) increased from 0.8 mmol/g-clay, however, further change in mw/mc was negligible. Concurrent with microbial reduction of structural Fe was a significant decrease in the swelling pressure (PI) of SWa-1: for example, when mw/mc = 1.2 (g/g), PI changed from 0.47 MPa at Fe(II) = 0.2, to 0.19 MPa at Fe(II) = 0.9 mmol/g-clay. Both biologically and chemically reduced smectites displayed lower values of mw/mc and a concurrent decrease in II as Fe(II) content increased, but the effect of Fe(II) on mw/mc was greater for the microbially reduced smectites at all applied pressures.