The principal objective of this investigation was to study the effect of microorganisms on rock wettability behavior. Crude oil was used to measure the contact angles both at 23 and 50 degrees C on solid rock samples. It was found that the bacteria obtained from crude oil had an effect on contact angles at both 23 and 50 degrees C. This effect depends on the type of nutrient used, type of rock sample, type of microorganism, and temperature at which the experiments were carried out.