Although studying teachers' online interactions has gained momentum in recent years, little is known about interactional development in online communities. The present study addressed this gap by exploring discursive change in second language teachers' interactions in an online professional development (PD) course structured around mobile phone usage. To this aim, the microgenetic analysis approach of sociocultural theory was adopted to track developments in teachers' interactions based on Goodall's (2000) skilled conversation threads. Data analysis revealed 152 skilled conversation exchanges in the online PD. Three themes were dominant among these exchanges and constituted most of the interactions: (1) Direct and indirect uses of mobile phones, (2) challenges and benefits of using mobile phones, and (3) teaching different language skills using mobile phones. Detailed analysis of the exchanges revealed that the teachers gradually changed in their perceptions about using mobile phones as well as the discourse they employed to represent their understandings. The findings are discussed in relation to teacher educators' role in facilitating online PD and the importance of developing a knowledge base that accounts for teachers' online membership.