  We show that a superconducting vortex in underdoped high T_c superconductors
could have an antiferromagnetic core. This type of vortex configuration arises
as a topological solution in the recently constructed SO(5) nonlinear sigma
model and in Ginzburg-Landau theory with competing antiferromagnetic and
superconducting order parameters. Experimental detection of this type of vortex
by \mu SR and neutron scattering is proposed.
