Microorganisms have been added to the subsurface in an attempt to enhance bioremediation. The transport, attachment and fate of these organisms as well as their possible contribution to remediation remain poorly understood. The effect of hydraulic conductivity and injection velocity on microbial transport through porous media was investigated. Glass chromatography columns were packed separately with clean quartz sand of two diameters (0.368 mm or 0.240 mm) and two hydraulic conductivities (1.37 × 10\sp−1 cm/sec and 3.65 × 10\sp−2 cm/sec respectively). Three injection velocities, 1.18 × 10\sp−3, 2.35 × 10\sp−3 and 4.73 × 10\sp−3 cm/sec were investigated. Microbial transport under the conditions tested was limited and could be predicted mathematically using a model based on filtration theory which incorporated particle trajectory analysis.