It is suggested that a useful component of a fertility simulation would be proportions of females sterile by age. Data on this phenomenon are rather limited, but they indicate that proportional sterility may not easily be described by a simple function. We propose that rates of becoming sterile may be adequately described by an exponential function utilizing proper parameter values. Such exponential-model rates form the basis for computation of a fecundity decrement table presented in this paper. Sterility proportions from the table are compared to some empirical data.