The present paper reports on a case study that explored the applicability of dynamic assessment (DA) for promoting learners’ word derivational knowledge in English as a second or a foreign language (L2). One learner’s performance on tasks assessing his word derivational knowledge was measured four times. The first two measurements were conducted before and after three weekly humanmediated DA sessions and the last two, which took place a year and a half later, before and after three weekly computerised DA sessions. Think aloud protocols and interviews were used to trace changes in the learner’s use of strategies and knowledge sources. The results revealed that following the dynamic assessment, the learner improved his performance and used strategies and knowledge sources more successfully. The findings have implications for designing dynamic tests of L2 English word derivational knowledge and for word derivational knowledge instruction.