A 22-year-old gas station clerk in Catonsville was seriously injured last night when a robber slashed him in the left side with a knife, Baltimore County police said.

The victim, who lives in Pikesville, was in serious condition last night after being flown to the Maryland Shock Trauma Center, a hospital spokeswoman said.

Police said the robber went to the clerk's booth at the Exxon station in the 6000 block of Baltimore National Pike about 7 p.m., where the victim and another clerk were working.

Without saying a word, he cut the 22-year-old clerk, police said. The robber then went inside the booth and ordered the other employee, a teen-ager, to turn over the money, police said.

The teen-ager stepped back from the cash register and the robber grabbed a handful of money and ran off in an unknown direction, police said.

The hold-up man was described as white, between 25 and 35, about 5 foot 7 and weighing 130 pounds. He had short brown hair and a thick mustache, and wore a white T-shirt and jeans.