Microbial activities directly affect the environmental quality of water, soil, and sediments. To improve our understanding of microbial attachment and transport in the subsurface, experimental studies were performed to evaluate bacterial adsorption and transport in two types of soil, Smolan (27% clay) and Haynie (5.5% clay) soils. Results indicate that bacterial breakthrough was slightly faster in columns with lower clay content and that the most rapid rate of bacterial adsorption occurred during the first 60 min of exposure.