The impact of computer simulations on the performance of 65 grade 11 learners in electromagnetism in a South African high school in the Mpumalanga province is investigated. Learners did not use the simulations individually, but teachers used them as an interactive demonstration tool. Basic concepts in electromagnetism are difficult to understand owing to their abstract nature. A non-equivalent control group design was followed. The performances of participants in the control group (N = 35) and experimental group (N = 30) were used. The t-test for independent samples showed that the experimental group achieved significantly higher scores on the post-test than the control group (t-statistic = 3.58, degrees of freedom = 63, p < 0.05). In the analysis of the test questions by topic, the experimental group also outperformed the control group consistently. The ability of computer simulations to aid learners' conceptual understanding was confirmed. Although the performance of the learners was established, the role of the teachers when selecting the most effective instructional designs to enable learners to understand the fundamental ideas in electromagnetism could not be overlooked.