  A measure of the average depression between Lyman absorption lines in the
spectrum of the faint quasar BR1202-0725 ($z_{em}=4.695$) is presented. The
relatively high resolution of the spectrum ($\sim 40$ km s$^{-1}$) allows the
selection of regions free of strong absorption lines in the Lyman alpha forest.
A reliable evaluation of the continuum shape is based on the careful flux
calibration and on the large wavelength interval covered (5000--9300 ~\AA). A
best estimate of $\tau _{_{GP}}\leq 0.02\pm 0.03$ has been found for the
Gunn-Peterson optical depth at the highest absorption redshift observed at this
resolution, $z \simeq 4.3$. The derived baryon density of the intergalactic
medium is $\Omega _{IGM}\mincir 0.01$ if the observed quasars are the major
contributor to the ionizing UV background flux. This limit, when compared with
the total baryon density deduced from the nucleosynthesis, could imply that
most of the baryons are already in bound systems at $z\sim 5$.
