We analyze all stock transactions executed by the universe of individual (or retail) investors of the Colombian Stock Exchange (5,380,810 trades performed by 42,211 individual investors between 2006 and 2016). Retail investors had negative abnormal returns on a gross excess return basis that ranged between 4% and 4.4% per year (depending on whether the alpha was estimated using the CAPM, Fama-French model or Carhart model). When transaction costs are considered, the underperformance of retail investors becomes even more pronounced, and the most active traders perform worse than less active traders even on a gross excess return basis. The underperformance of retail investors can be explained by their bad timing but only prior to the bankruptcy of Interbolsa, the largest stock brokerage house in Colombia at the time (2012). Once we control for the number of trades and other variables, we find that retail investors present in the market for a longer period of time and trading more actively outperform the other investors (on both a gross and net basis).