The Ames Test Curve Fitting program is a Fortran program which fits a nonlinear dose-response curve to count (Poisson) data. The program was written specifically for analyzing Ames test data, although it is possible that the program could have other applications. The curve fitted is of the form (a+exp(b+clogX)exp(-dx)) where a, b, c and d are parameters to be estimated. If the parameter d is very near zero, then d is dropped from the model, and a reduced model is fitted. Tests of goodness-of-fit and for the significance of the parameters are made using likelihood ratio tests. The program has optional graphics capability for producing plots of the data and the estimated curve.