Temporal variations in soil hydraulic properties due to biological activity are not well understood. Research has demonstrated reductions in saturated hydraulic conductivity with prolonged water saturation and nutrient injection, but much less is known regarding the effects of microbial activity on unsaturated soil hydraulic properties. The purpose of this study is to quantify the changes in the unsaturated hydraulic conductivity K(Θ)and moisture retention Θ(h) accompanying stimulation of the microbial population under unsaturated conditions. The hydraulic properties and microbial activity of replicate soil cores were monitored for several months following addition of glucose plus nutrient solution and incubation at 30 C. Soil hydraulic functions were measured at several elapsed times using a continuous flow method in combination with inverse numerical analysis of Richards' equation. Both draining and wetting hydraulic functions were measured so that the effect of microbial activity on soil water hysteresis can be explored. Concurrent with the hydraulic property measurements, total and active fungal and bacterial numbers were estimated on parallel soil cores using CO2 evolution epifluorescence microscopy. Changes in the hydraulic conductivity function, pore-size distribution, and water retention as a result of microbial growth will be discussed.