Thin current sheets, with thicknesses of the order of a few ion gyroradii, are believed to exist quite commonly at the Earth's magnetopuase as well as in the magnetotail current sheet just prior to substorm onset. Analytical theory predicts that sheets of this kind can become the locus of an ion-tearing instability. We use a modified hybrid simulation code that treats the ions fully kinetically and the electrons as a fluid with a model for the full pressure tensor to study the ion tearing mode development in one-dimensional current sheets of various thicknesses, with varying initial electron to ion temperature ratios. Further, we investigate the electron anisotropy dependence of the ion tearing mode growth rate and compare the growth rates found in the numerical simulations to linear growth rates for the pure ion-tearing instability derived from analytic theory. Estimates of the numerical accuracy of the simulations and of possible electron distribution functions resulting in the modeled anisotropies are also presented. ¿ American Geophysical Union 1994