A laboratory experiment was designed to increase the understanding of the geoelectric effects of microbial degradation of hydrocarbons. Eight large columns were paired to provide a replicate of each of four experiments. These large-volume columns contained 'sterilized' soil from a well-studied field site and 25% BH nutrient solution. Two columns contained diesel fuel, two contained microbes, and two contained both microbes and diesel. The remaining two columns were control columns, with only soil and 25% BH solution. A fixed vertical array of electrodes was mounted inside the wall of each column. Apparent resistivity data were collected from each column at 2-cm intervals every 10 day for more than 18 months. These data show large changes in the conductivity of the eight nonaqueous phase liquid (LNAPL) impacted zones, but relatively small changes in the control columns.