This paper examines the stock market reaction to annual earnings information releases using data on the Nigerian Stock Exchange. Using the event study method, the speed of reaction of the market to annual earnings information releases for a sample of 16 firms listed on the exchange is tested. Significant abnormal price reactions around earnings announcements suggest the earnings announcements contain value-relevant information. We find that the magnitude of the cumulative abnormal returns is dominated by significant reactions 20 days before the earnings release date which suggests that a portion of the market reaction may be due to private acquisition and, possibly, abuse of information by insiders. The persistent downward drift of the cumulative abnormal returns, 20 days after the announcement, is inconsistent with the efficient markets hypothesis, and therefore suggests that the Nigerian stock market does not efficiently adjust to earnings information for the sample firms within the study period.