Agile methods such as extreme programming are becoming increasingly important for the rapid development of software applications. They proofed to be applicable and beneficial for a variety of domains. Nevertheless, due to the iterative nature of these approaches, software developers focus on functional aspects and tend to neglect nonfunctional characteristics, like usability. This paper addresses this issue by providing an extension suggestion for extreme programming that helps the on-site customer and development team to take into account usability. Usability criteria are attached to user stories, and a new type of user stories, called usability stories are introduced. This also influences the way on-site customers perform the acceptance test. We present first results of a case study where small changes to extreme programming have been introduced. The benefits were an increased perception of usability due to the integration into the user stories and an early evaluation of usability criteria in the acceptance test. Eventually, these measures lead to a high usability of the software system.