  We report an experimental demonstration of single-photon switching in
laser-cooled $^{87}$Rb atoms. A resonant probe pulse with an energy per unit
area of one photon per $\lambda^2/2\pi$ propagates through the optically thick
atoms. Its energy transmittance is greater than 63% or loss is less than
$e^{-1}$ due to the effect of electromagnetically induced transparency. In the
presence of a switching pulse with an energy per unit area of 1.4 photons per
$\lambda^2/2\pi$, the energy transmittance of the same probe pulse becomes less
than 37% or $e^{-1}$. This substantial reduction of the probe transmittance
caused by single switching photons has potential applications in
single-photon-level nonlinear optics and the manipulation of quantum
information.
