We present the first measurement of pseudorapidity densities of primary cha
rged particles near midrapidity in Au + Au collisions at root S-NN = 56 and
130 GeV. For the most central collisions, we find the charged-particle pse
udorapidity density to be dN/d eta/(/eta/<t) = 408 +/- 12(stat) +/- 30(syst
) at 56 GeV and 555 +/- 12(stat) +/- 35(syst) at 130 GeV, values that are h
igher than any previously observed in nuclear collisions. Compared to proto
n-antiproton collisions, our data show an increase in the pseudorapidity de
nsity per participant by more than 40% at the higher energy.