Aerated biological reactors at a laboratory scale, fed continuously with a model wastewater system, were used to obtain design parameters of an activated sludge process for the wastewater treatment of a potato processing plant. Experiments were performed using different dilution rates (D) ranging between 0.01 and 0.13 h(-1) to determine parameters such as: maximum specific microbial growth rate (mu(max)), saturation constant (K-s), coefficient of microbial decay (k(d)) and true biomass yield (Y-x/s) Design parameters were calculated from the mass balances of substrate consumption and biomass production under steady state, considering a Monod-type equation to represent the effect of substrate concentration on biomass production rate. The values of Y-X/S and k(d) were obtained from the linear plot of q(s) (specific rate of substrate consumption) vs. D. Experimental data regression led to y(X/S) = 0.45 gVSS/g COD and k(d) = 0.024 h(-1). By non-linear regression of q(s) vs. substrate concentration (S-s) data the following values were obtained mu(max) = 0.09 h(-1) and K-s = 0.006 g/l. Critical dilution rate was also calculated; it corresponds to the D value for which the cells of a reactor operating under such conditions are destroyed or are eliminated from the system at the same rate than they are produced.