In the domain of biotechnology array-based methods are used to gain rapid access to genetic information based on the signals of the individual array elements (spots). For an automated analysis of the spots it is necessary to fit a grid to the spots in the digital image in order to represent the array distortions that may occur in the course of the experiment. In order to make the grid fitting problem tractable in a certain class of experiments spot arrays contain a sub-grid of guide spots with a known signal characteristic. We present an automatic grid fitting method for spot array images containing guide spots. Our approach uses simple image processing methods and takes into account prior knowledge inherent in the imaging process.