In this paper, we examine how the students' writing culture changed over time, while using computer mediated communication in two Computer Science courses. We investigate how the students started using mostly casual language in online messages and posts at the courses' e-fora and subsequently, how they adopted a more formal communication with their professors and peers over time. The only driving force for this change was the positive examples from the professors' formal replies, since students never received criticism-or any kind of other feedback-for their casual messages or replies. For this study, we have collected and analysed a total of 315 messages from these two courses, send by 75 individual students. These messages were classified into two categories 'casual' and 'proper', using a set of strict criteria that the fulfilment of all was required for a message to be classified as 'proper'. The results of the analysis of these messages revealed that, using only the professor's formal responses as a positive example, students changed significantly their writing culture over time and, while they had started communicating using casual language at the beginning of each course, after a few weeks they had adopted a more suitable language.