This work presents an Extended Kalman Filter (EKF) for estimating the kinetic rates in bioprocesses. The unknown kinetic rates are modeled as the output of a linear system persistently excited. The gain of the filter is designed by the Ricatti differential equation. The stability and convergence are analyzed using Lyapunov functions. To illustrate the application of this observer, the estimation of the kinetic rates of a single microbial culture is presented.