Research syntheses have demonstrated that morphological instruction can improve the literacy skills of poor readers and spellers. However, studies have used a wide variety of training methods. Questions remain about what type of morphological instruction is most effective and under which circumstances. In this study, we conducted a randomized controlled trial evaluating the effectiveness of Structured Word Inquiry for poor readers and spellers. Structured Word Inquiry teaches students the logic of the English spelling system via instruction in morphology and etymology. Students in grades 3 and 5 with poor reading and spelling skills were randomly assigned to receive either Structured Word Inquiry instruction or a comparison instruction method involving robust vocabulary instruction and reciprocal teaching. Instruction was delivered by teaching assistants over the course of a full school year. After training, there were few differences between the groups in terms of literacy or vocabulary skills. However, teaching assistants found Structured Word Inquiry instruction challenging to deliver, which is likely to have impacted the results. Our findings have implications for the nature and content of morphological instruction for poor readers and spellers, and for future attempts to scale up the delivery of morphological interventions.