We show that a superconducting vortex in underdoped high T-c supercond
uctors could have an antiferromagnetic core. This type of vortex confi
guration arises as a topological solution in the recently constructed
SO(5) nonlinear a model and in Landau-Ginzburg theory with competing a
ntiferromagnetic and superconducting order parameters. Experimental de
tection of this type of vortex by muon spin resonance and neutron scat
tering is proposed.