In this paper, we focus on the usability testing of mobile applications, particularly native (instead of Web based) applications. We envision a system that can automatically collect user interface (UI) events as the user interacts with the mobile application. The collected UI data will then be uploaded to a remote server for either automated or manual usability analysis. This kind of system can complement traditional laboratory testing, and we believe it will be particularly useful to deploy for field-based usability testing. For many mobile application developers, it is often too costly to conduct extensive laboratory-based usability testing and we anticipate that the system described in this paper will be an indispensable toolkit for low-cost usability analysis.