  A measurement of the derivative (d ln F_2 / d lnx)_(Q^2)= -lambda(x,Q^2) of
the proton structure function F_2 is presented in the low x domain of deeply
inelastic positron-proton scattering. For 5*10^(-5)<=x<=0.01 and Q^2>=1.5
GeV^2, lambda(x,Q^2) is found to be independent of x and to increase linearly
with ln(Q^2).
