Tim Wallach hit a 1-1 pitch off Jesse Orosco for a home run in the ninth inning at Montreal last night and ended the New York reliever's nine-game winning streak as the Expos beat the Mets, 10-9, and regained first place in the NL East Division.

With Philadelphia's loss at Pittsburgh, the Expos moved a half-game in front of the Phillies with the Pirates one game out and St. Louis 1 1/2 behind.

Wallach's 19th homer of the year, over the left field fence, came with one out and handed Orosco (13-6) his first defeat since July 19; his streak ended one shy of the Mets' record set by Tom Seaver in 1969. Jeff Reardon pitched the ninth inning for the victory, bringing his record to 6-8.

New York tied the score, 9-9, with two out in the ninth on a two-run double by Hubie Brooks.

After Mookie Wilson led off the game with a home run to give New York a 1-0 lead, Montreal scored four runs in the bottom of the first on Gary Carter's sixth career grand slam. Tim Raines walked, Manny Trillo doubled, Al Oliver walked and Carter hit his 16th home run of the season and first since Aug. 17.

In the second inning, Trillo doubled and Andre Dawson hit his 30th home run, for a 6-1 lead. Carter's second homer came in the fourth and made the score 7-1.

New York scored four runs in the fifth, but in the home half, Montreal made it 9-5 on Mike Fuentes' first major league hit followed by Raines' ninth home run.

In the Mets' eighth, George Foster singled and Darryl Strawberry hit his 23rd home run. Strawberry tied John Milner's club record for home runs by a left-handed batter and Steve Henderson's club rookie RBI record of 65.