In this article, we describe the development of a mobile butterfly-watching learning (BWL) system to realize outdoor independent learning for mobile learners. The mobile butterfly-watching learning system was designed in a wireless mobile ad-hoc learning environment. This is first result to provide a cognitive tool with supporting the independent learning by applying PDA with wireless communication technology to extend learning outside of the classroom. Independent learning consists of self-selection, self-determination, self-modification, and self-checking. To support independent learning, we designed useful butterfly-watching learning subsystems to offer the self-selection, self-determination, self-modification, and self-checking functionality. The study sample consisted of 24 five-year students who enrolled in natural science course in the Fall of 2003 in Taiwan. The study lasted fourth months (one semester). During the butterfly-watching activity, each learner arbitrarily took a distinct butterfly picture and entered it into his own PDA to acquire the butterfly knowledge anytime and anywhere through wireless communication. The potential impacts, limitations, and lessons learned from this study are discussed from both the educational and technological points of view. Mobile outside classroom learning potentially extends the scope of learning activities and with more flexibility than in traditional school curriculum. This work shows that the concept of applying PDA with wireless communication technology is a "new cognitive learning tool" to personally access, analyze, interpret, and organize their personal knowledge anytime and anywhere. (Contains 14 figures and 1 table.)