This study investigated the effectiveness of different types of feedback content (elaborate versus global) and feedback timing (immediate versus delayed) for learning genetics in a web-based learning environment as a function of learners' prior knowledge. It was hypothesized that learning outcomes of students with low prior knowledge would be fostered by immediate elaborate feedback, whereas those of students with more prior knowledge would be enhanced by delayed global feedback. Students' perceptions of the feedback they received were explored. Results showed a significant positive effect of global feedback on learning outcomes for higher prior knowledge learners, although those who received elaborate feedback gave a higher appreciation rating. The findings are discussed in terms of implications for the design and delivery of feedback in web-based learning environments. (Contains 2 tables.)