The general, energy - momentum tensor for a dynamical, string fluid in general relativity is presented using the Ray - Hilbert variational principle. The calculations are given for both the standard and the extended thermodynamics versions in which the latter includes the string as thermodynamic variables. In general relativity, it is shown that the string does not Fermi - Walker transport. Examples of solutions to the field equations are given and compared with solutions obtained from the postulated energy - momentum tensor of Letelier.