The study verifies, between march 2005 and march 2006, if there is any difference between the stock yields of a sample of companies listed in Bovespa that adopt the principles of Corporate Governance, in different levels, in relation to the companies of the traditional market. The data were extracted from Economatica’s database; 130 non-financial companies with higher liquidity stock were selected, of which 65 adopt Corporate Governance practices, and 65 operate according to the traditional market. To conduct the study, the Analysis of Variance (ANOVA) and Student’s t test were used. The results show that the mean yield of companies that adopt different Corporate Governance levels was superior to that of the companies which did not adopt these practices. These results demonstrate that a higher level of transparency, and consequently lower information asymmetry, can lead to a better company performance.