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A measure of the average depression between Lyman absorption lines in thespectrum of the faint quasar BR1202-0725 ($z_{em}=4.695$) is presented. Therelatively high resolution of the spectrum ($\sim 40$ km s$^{-1}$) allows theselection of regions free of strong absorption lines in the Lyman alpha forest.A reliable evaluation of the continuum shape is based on the careful fluxcalibration and on the large wavelength interval covered (5000--9300 ~\AA). Abest estimate of $\tau _{_{GP}}\leq 0.02\pm 0.03$ has been found for theGunn-Peterson optical depth at the highest absorption redshift observed at thisresolution, $z \simeq 4.3$. The derived baryon density of the intergalacticmedium is $\Omega _{IGM}\mincir 0.01$ if the observed quasars are the majorcontributor to the ionizing UV background flux. This limit, when compared withthe total baryon density deduced from the nucleosynthesis, could imply thatmost of the baryons are already in bound systems at $z\sim 5$.Comment: 11 pages, 3 figures available on request, LaTex file, accepted for  pubblication in ApJ

The Gunn-Peterson effect in the spectrum of the Z = 4.7 QSO 1202-0725: The intergalactic medium at very high redshifts