  The mid-infrared spin-wave spectrum of antiferromagnetic
YBa$_2$Cu$_3$O$_{6.0}$\ was determined by infrared transmission and reflection
measurements ($\bbox{k} \!\! \parallel $c) at $T\!=\!10\!$~K.\@ Excitation of
single magnons of the optical branch was observed at
$E_{\text{op}}\!=\!178.0\!$~meV.\@ Two further peaks at $346\!$~meV
($\approx\!1.94\,E_{\text{op}}$) and $470\!$~meV
($\approx\!2.6\,E_{\text{op}}$) both belong to the two-magnon spectrum. Linear
spin wave theory is in good agreement with the measured two-magnon spectrum,
and allows to determine the exchange constant $J$ to be about $120\!$~meV,
whereas the intrabilayer coupling $J_{12}$ is approximately $0.55\,J$.
