We consider a class of ratio-product estimators for estimating a finite population mean. The asymptotically optimum estimator in the class is identified, along with its approximate mean-square error.This estimator requires prior knowledge of the parameter C D ρC y =C x , where ρ is the correlation coefficient between the study variate y and the auxiliary variate x, and C y and C x are coefficients of variation of y and x respectively. If C is unknown in advance, then it can be replaced by its consistent estimateĈ, with the resulting estimator known as an 'estimator based on the estimating optimum'. It is shown that, to the first order of approximation, both estimators have the same mean-square error, and that they are generally more efficient than the usual ratio and product estimators.