In the present study, we propose a virtual boundary method for simulation of massive inextensible flexible
strings immersed in viscous fluid flow. The fluid motion is governed by the Navier-Stokes equations and a
momentum forcing is added in order to bring the fluid to move at the same velocity with the immersed surface. A
massive inextensible flexible string model is described by another set of equations with an additional momentum
forcing which is a result of the fluid viscosity and the pressure difference across the string. The momentum forcing
is calculated by a feedback loop. Simulations of several numerical examples are carried out, inlcuding a hanging
string which starts moving under gravity without ambient fluid, a string swimming within a uniform flow and a
uniform flow over two side-by side strings. The numerical results agree well with the theoretical analysis and
previous experimental observations. Preliminary results of a swimming elongated fishlike body will also be presented.