In thermal gradients, pseudoplasmodia of slime molds and starved nematodes have been reported to move away from a temperature near their acclimation temperature. The consequences of this behavior are not clear in the thermal environment of soil where typically there are waves of alternating high and low temperatures propagating down into the soil with little difference in temperature averaged over time. Computer modeling of this situation demonstrates complex movements with important consequences. The organisms move upward during two intervals during a day and downward during the intervening intervals. The upward and downward movements do not balance. The net movement may be in either direction depending on the specific values of certain physiological parameters. Thus, thermotaxis can lead to a net change in depth even though the average temperature is the same everywhere. Organisms moving faster than the rate of penetration of the thermal wave (2–3 cm/h) will follow it into the interior. Also, organisms that avoid a temperature outside the range of thermal variation, will have a net movement away from the surface. However, slow moving organisms that avoid a temperature near the mean temperature move toward the surface and will reach it in twice the time it would take if always moving toward the surface. The optimal rate of movement to reach the surface in minimum time is about 0.8 cm/h. Thermal acclimation can increase the efficiency of moving toward the surface. At the rate of locomotion of slime molds (0.1–0.2 cm/h), the optimal rate of acclimation is about 0.3/h, which is approximately their actual rate of acclimation. These results suggest that thermotaxis can be used by simple organisms to move toward or away from an interface between media of different thermal properties even though there is no gradient in average temperature.