Boullion, who is the official test driver for the Williams- Renault team, clocked 1min 21.97sec, just 0.06 of a second slower than the time Schumacher set in his Benetton-Renault.
Such was the admiration for Boullion's time that the Benetton team tried in vain throughout the afternoon to post a faster lap and extend their advantage.
Finland's Mika Hakkinen was third fastest in the McLaren-Mercedes, but he was unable to approach the pace of the quicker cars. His fastest lap of 1:24.42 was almost three seconds off the pace.
Damon Hill will take over the Williams-Renault car today for the final two days of testing. Having been out-paced by Schumacher during last weekend's Spanish Grand Prix, in which he finished fourth, he will be keen to re- affirm his position as world championship favourite before the next race at Monaco on 28 May.