AbstractField trials of microbially induced desaturation (MID) were conducted at two sites in Portland, Oregon underlain by liquefiable fine-grained soils. MID is an emerging method for mitigating the potential for triggering liquefaction. MID treatment stimulates native denitrifying microbes with a solution containing nitrate, as well as other substrates and nutrients. An end product of the denitrification reactions is nitrogen gas, which displaces soil porewater and in turn reduces the in situ degree of saturation (Sr). Because during cyclic loading desaturated soils produce less excess porewater pressure than saturated soils, MID can mitigate the potential for triggering liquefaction. Monitoring for the two field trials was performed to evaluate the MID treatment performance and the associated subsurface desaturation. Monitoring data included seismic wave velocities measured with crosshole and downhole techniques, embedded in situ moisture and electrical conductivity sensors, water chemistry measurements, and recovery and testing of samples for changes in soil properties. Monitoring data were collected pretreatment, during treatment, and post-treatment, and then interpreted to evaluate the effectiveness of MID for reducing Sr in fine grained, low plasticity silts in the two distinct sites. Despite geotechnical site characterization data that show the field trial sites have distinct geotechnical characteristics, including interbedding, that affect liquefaction susceptibility and MID treatment application, results indicate liquefiable soil at both sites was successfully desaturated and that the desaturation persisted for at least 92 days post-treatment.