This study seeks evidence supporting the existence of market efficiency and exchange rate sensitivity on stock prices in the Johannesburg stock exchange (JSE). The sample includes the daily price indices of all securities listed on the JSE, and the exchange rate of the USD/Rand for the period since January 2000 to December 2004. The results from the unit root test, the ADF test and the causality test at the Granger sense provide evidence that the Johannesburg stock exchange (JSE) is informationally efficient. It has a long run co-movement with exchange rate, and long run equilibrium or steady state. Hence, in JSE there is a strong possibility that foreign direct investors and forex market traders cannot influence and gain abnormal extra benefits by using exchange rate mechanism or by using exchange rate to forecast stock prices in the market. So, JSE is semi-strong form efficient. Through co-integration test, this paper gives more insight on the concept of market efficiency and the reliability of the results. These results are important to security analysts, investors, and security regulatory exchange bodies in policy making decision to improve the market conditions.