In this paper, we present a case-study on developing an adaptive computer assisted language learning software built for mobile devices. Our application, called Mobile Adaptive CALL (MAC) is aimed at helping Japanese-English speakers in perceptually distinguishing the non-native /r/ vs. /l/ English phonemic contrast with a view to improving their discrimination. The software is adaptive and more practice is given for the learner on contrasts that are most difficult for them, and learners may choose their preferred level of difficulty. MAC is implemented in Java (J2ME), allowing the software to be used on a wide range of mobile devices including most recent mobile phones. In our case-study, we give an overview of the software, results from initial user studies and suggest some considerations for designing and implementing mobile learning applications for speech/audio language training.