We study the causes and the consequences of two regulatory changes affecting the competition between the transparent Parquet and the OTC-like Coulisse markets in Paris at the turn of the 20th century. First, we provide evidence supporting the interest group theory to explain regulatory changes. By using these changes as natural experiments, we show then that competition widens bid-ask spreads while monopoly makes them narrower. These results are in line with recent literature questioning the effects of “dark” competition: a transparent monopoly could be more effective than competition if the latter involves opaque markets.