The CXTFIT Code for Estimating Transport Parameters from Laboratory or Field Tracer Experiments. The software package includes a modified and updated version of the CXTFIT code of Toride et al.[1995] for estimating solute transport parameters using a nonlinear least-squares parameter optimization method. This code may be used to solve the inverse problem by fitting a variety of analytical solutions of theoretical transport models, based upon the one-dimensional advection-dispersion equation (ADE), to experimental results. The program may also be used to solve the direct or forward problem to determine concentrations as a function of time and/or position. Three different one-dimensional transport models are considered: (i) the conventional equilibrium ADE; (ii) the chemical and physical nonequilibrium ADEs; and (iii) a stochastic stream tube model based upon the local-scale equilibrium or nonequilibrium ADE.