An improved and simplified version of the string method in collective variables for computing minimum free energy paths is proposed. The string is discretized into a finite number of images and in the new method these images are evolved concurrently with replicas of the original system while keeping their inter-distances equal via reparametrization. There is no need to compute the mean force by time-averaging, nor to project the force perpendicularly to the string. In this Letter, the algorithmic aspects of the on-the-fly string method are presented in detail and the method is tested on the solvated alanine dipeptide molecule. (C) 2007 Elsevier B.V. All rights reserved.