Zusammenfassung: Equations describing biological phenomena frequently have parameters entering nonlinearly. A highly efficient method for detemining the values of parameters in such equations that will give the best least-squares fit to a set of data is Marquardt´s algorithm. A step-by-step derivation of the algorithm is given in terms that require only the knowledge of elementary calculus and matrix algebra. As an example, the method is used to fit the logistic equation to a set of data.