Fractal theory was applied to describe fingering structure and to estimate the effective surface tension at the wetting front during infiltration. A scale length of mean pore size was used to estimate the microscopic system length of fingering. After bulk surface tension was replaced by effective surface tension in a linear stability theory, theoretical maximum wavelengths agreed with those of experimental results when fingering developed in quartz sand beneath Oakley sand, but not beneath Yolo clay. ¿ American Geophysical Union 1993