This article discusses learning styles as a criterion of adaptation of a course online. A first step is to choose the model of learning styles. The identification of these styles is performed by a dedicated questionnaire. On the other hand, learning activities are designed to reflect the dimensions related to learning styles. Finally, the presentation of these activities is managed by a probabilistic adaptation module. Based on the methods and techniques proposed for modeling and adaptation, we designed an adaptive hypermedia system that focuses on learning styles. The approach was validated experimentally and the results are encouraging.