I use experimental asset markets to analyze trading under di¤erent transparency and information settings. I …nd that both liquidity and informed traders use undisclosed orders to compete for liquidity provision. In opaque markets, traders increase aggressiveness to improve execution probability. Without information friction, market opacity enhances liquidity, especially toward the end of trading, and is bene…cial for liquidity traders.Under informed trading, adverse selection drives market outcomes mainly around news announcements. Monopolistic insiders exploit opacity at the expense of large liquidity traders. Opacity does not a¤ect informational e¢ ciency with a monopolistic insider, but value discovery is faster when informational rents are shared.(JEL classi…cation: C91, C92, G28)