The vortex system in a high-T-c superconductor has been studied numeri
cally using the mapping;to 2D bosons and the path-integral Monte Carlo
method. We find a single first-order transition from an Abrikosov lat
tice to an entangled vortex liquid. The transition is characterized by
an entropy jump Delta S approximate to 0.4k(B) per vortex and layer (
parameters for YBa2Cu3O7) and a Lindemann number c(L) approximate to 0
.25. The increase in density at melting is given by Delta rho approxim
ate to 6.0 x 10(-4)/lambda(T)(2). The vortex liquid corresponds to a b
osonic superfluid, with rho(s), = rho even in the limit lambda --> inf
inity.