The aim of the present paper was to analyse relative errors and variability coefficients for estimated values of maximum specific growth rate (u_max) and saturation constant (K_S) in the Monod’s growth model in the case when these values are determined on the basis of data from steady states of two chemostatic continuous cultures, and also on the basis of the above to propose the procedure for the dilution rate selection so that the analysed quantities attained minimum values. The above-mentioned relative errors and variability coefficients were analysed at assumption that the relative errors of determined values used for estimation, i.e. the dilution rate and substrate concentration in fermenter, are constant. It as found that in the analysed estimation method both relative error so as variability coefficient for saturation constant are higher than for maximum specific growth rate and that the relative errors and variability coefficients values for both estimated parameters are then minimum when one culture is carried on at possible low dilution rate and the second at the dilution rate close to the washout rate. It was also established that carrying on one continuous culture at the fixed in advance dilution rate it cannot be possible to always select the dilution rate of the second culture, so that the simultaneous minimization of relative error for u_max and K_S was possible. It also relates to the minimization of variability coefficient.