  We report an indication that the elastic scattering rate of solar $^8$B
neutrinos with electrons in the Super-Kamiokande detector is larger when the
neutrinos pass through the Earth during nighttime. We determine the day/night
asymmetry, defined as the difference of the average day rate and average night
rate divided by the average of those two rates, to be
$(-3.2\pm1.1(\text{stat})\pm0.5(\text{syst}))\%$, which deviates from zero by
2.7 $\sigma$. Since the elastic scattering process is mostly sensitive to
electron-flavored solar neutrinos, a non-zero day/night asymmetry implies that
the flavor oscillations of solar neutrinos are affected by the presence of
matter within the neutrinos' flight path. Super-Kamiokande's day/night
asymmetry is consistent with neutrino oscillations for
$4\times10^{-5}$eV$^2\leq\Delta m^2_{21}\leq7\times10^{-5}$eV$^2$ and large
mixing values of $\theta_{12}$, at the $68\%$ C.L.
