A scalar field with an exponential potential allows a scaling solution wher
e the density of the field follows the density of the dominating fluid. Suc
h a scaling regime is often used as an important ingredient in many models
of quintessence. We analyze the evolution of perturbations while the backgr
ound follows the scaling. As a result, the perturbed scalar field also scal
es with the perturbed fluid, and the perturbations accompany the adiabatic
mode as well as the isocurvature mode between the fluid and the field.