To trace the origin of photocurrent (PC) in semiconductor superlatices, we have studied the PC spectra as a function of the applied electric field in a novel compositionally graded superlattice, where each well produces a separate peak. The sample consisted of 15 quasiperiods of 40/20-Å Al$x$Ga${1-x}$As/Al$y$Ga${1-y}$As [$x = 0.03(n-1)$, $y = x + 0.3$, and ($n$ = 1, …,15)]. We show that the PC is due to the transport of electrons with no significant contribution from the holes. At low electric field, when the transport is very slow, the PC originates from a few wells close to the substrate, thus providing local information, unlike optical absorption. As the field increases the active region expands and finally covers the whole superlattice.