  We show that the cosmic star formation rate per comoving volume should
exhibit a distinct drop around the reionization redshift, when the HII regions
in the intergalactic medium around individual ionizing sources first
overlapped. The drop results from the increase in the temperature of the
intergalactic medium as it was photo-ionized, and the consequent suppression of
the formation of low-mass galaxies. We show quantitatively that the detection
of this drop, which is marked by a corresponding fall in the number counts of
faint galaxies, should become feasible over the coming decade with the Next
Generation Space Telescope.
