Plasma surface waves are excited by a high frequency field in a semi-infinite plasma with a plane boundary when omega 0- omega p approximately=+or-( omega p/ square root 2) and omega 0- omega p approximately=+or-( omega p square root 2)- omega 0 ( omega 0 being the frequency of the high frequency field and omega p the plasma frequency). The instabilities, in effect, take place at (nc/n0)=3, 8.5*10-2, 0.72 and 2*10-2, where nc and n0 are the critical and the particle densities respectively. Threshold conditions are derived for plasmas with and without a low density electron beam. In the absence of the beam the thresholds are more than the relativistic limit while its presence reduces the thresholds considerably under certain conditions.