Incorporating usability testing into the development cycle of mobile survey applications is a very important approach to human-centered design. The U.S Census Bureau is implementing the agile software development methods for its Web survey instruments and tools. The agile practice provides us with both challenges and opportunities for usability testing. On one hand, it allows for usability feedback and recommendations to be considered in the early iterative phase of development rather than at the end where changes may be difficult to implement. On the other hand, the amount of time that can be afford for conducting usability testing is getting even less. In this talk, we will use the project, the Census Operations Mobile Platform for Adaptive Services and Solutions (COMPASS), as an example to illustrate the role of usability testing during the agile development cycle, obstacles or challenges, and impact on the iterative design of Web-survey applications.