The propagation rate of a natural hydraulic fracture is limited by the rate that fluid flows from the saturated rock into the void space created by fracture growth. Unlike induced hydraulic fractures, natural hydraulic fractures cannot be modeled by specifying rates of fracture growth or fluid flow into the fracture as boundary conditions. Numerical solutions of the governing equations for natural hydraulic fracture growth in a poroelastic medium indicate that growth rates are primarily controlled by the hydraulic conductivity &kgr;, the storage S'p, and the initial flaw length 2a0. Computationally more efficient models which partially and completely decouple material stresses from fluid pressures give similar results. Results for several rock types indicate that, although the rate of fracture propagation is limited by fluid flow, fracture growth still accelerates. Results are generalized in dimensionless plots of fracture length versus time for various values of the dimensionless parameter ϕ=(1-&ngr;)/(GS'p), where G is the shear modulus and &ngr; is the drained Poisson's ratio. ¿ American Geophysical Union 1994