The larval ventral belts of Drosophila consist of six to seven rows of denticles that are oriented, some pointing forwards, some backwards. We present evidence that denticle orientation is determined almost entirely by Dachsous and Fat, one of two planar cell polarity systems. If we change the distribution of Dachsous we can alter the polarity of denticles. We suggest that the orientation of the individual denticle rows, in both the anterior compartment (which mostly point backwards) and the posterior compartment (which point forwards), is determined by the opposing slopes of a Dachsous/Fat gradient. We show, by altering the concentration gradients of Dachsous during development, that we can change the polarity of the denticles made by larval cells as they progress between the first and third larval instars without mitosis.