Archipelago, the third-largest ECN, will use SuperMontage only until its own Archipelago Exchange is ready to take on electronic trading in Nasdaq stocks. Currently Archipelago trades stocks listed on the New York Stock Exchange, American Stock Exchange and Pacific Exchange. Using the ADF would have caused major work, says Archipelago: "It did not make sense for us to put the work into the ADF, and then a month later launch the Nasdaq stocks on our exchange," says a spokeswoman.

Instinet and Island have not yet come to a settled view of the merits of ADF, and Island will continue to use the Cincinatti Exchange at least until the two firms integrate their matching engines.

The ADF was created in response to an SEC requirement that there should be a competitor to SuperMontage, but does not have any order routing or order-execution capabilities; and so far at least Instinet is the only ECN to use it. An Instinet official recently said the firm decided not to post in SuperMontage, because to do so would be akin to "subsidizing a competitor."