Financial market management and different trading strategies.

Summary For a long time, the concentration of exchanges was seen as a necessary condition for improving liquidity. However, the development of new trading systems (MTFs and ATSs) has shown that concentration does not meet investors' needs in terms of liquidity, transaction costs and speed of execution. Based on this observation, the new European regulations and the London equity market are working to better meet these needs by improving competition between the various markets. One of the consequences of these changes is the fragmentation of exchanges. This research aims to study the original case of the London Stock Exchange (LSE). Three objectives are specifically addressed. The first objective is to study the effects of the introduction of a passive order crossing system (the "Crossing Network" called POSIT) on the transaction costs as well as on the information asymmetry costs of the LSE. Does the introduction of CN have positive or negative effects on the central market? Does it improve competition in the central market? The second objective is to examine the assimilation of private information by prices. Is private information integrated in the same way in a price-driven market such as SEAQ as in an order-driven market such as SETS? Finally, the last objective is to identify the order size used by informed investors on SETS, SEAQ as well as for the FTSE 250 stocks traded in SETS.