Purpose: The purpose of this paper is to explore the role of community of practice in sustaining teachers' participation in a blended (face-to-face and online) professional development course. Design/methodology/approach: A longitudinal multiple-case study methodology was used in researching groups of five teachers in Australia and four teachers in the UK. The two groups independently participated in an initial face-to-face training day and then completed the professional development course via an online learning environment (Blackboard). The course was designed to facilitate community of practice cohesion. Data collection included surveys, data mining of online activity, discussion forums and e-mails, and semi-structured interviews. Findings: The paper finds that the participants of both case studies demonstrated sustained engagement for more than twice the minimum requirement. While the Australian case study was sustained through a community-defined regime of participation, the UK case study increasingly relied on the course facilitator to broker both practices and a community rhythm by which they could participate. Sustained participation in the Australian case study was supported by mutuality as seen in reciprocity of interaction and social engagement. Originality/value: Transformative professional development is dependent on a number of design principles, including that it needs to be sustained over time. The literature reveals that neither face-to-face nor online professional development, per se, sustains engagement. This research demonstrates that teacher participation can be sustained by designing for community of practice cohesion, in a blended mode of delivery with small groups of participants. (Contains 4 figures.)