We have measured the sideward flow of neutral strange (K-s(0)) mesons in 6A
GeV Au + Au collisions. A prominent antiflow signal is observed for an imp
act parameter range (b less than or equal to 7 fm) which spans central and
midcentral events. Since the K-s(0) scattering cross section is relatively
small in nuclear matter, this observation suggests that the in-medium kaon
vector potential plays an important role in high density nuclear matter.