LOWELL — Two men accused of possessing five pounds of marijuana also are facing an unusual charge: They didn’t pay state tax on their pot.

Virak Paul Prum, 25, and Bounnee Lakongpheng, 22, both of Lowell, are being charged under an obscure section of a 2010 law revision. The Massachusetts Controlled Substance Tax states: “No dealer shall possess any marijuana or controlled substance upon which a tax is imposed … unless the tax has been paid on the marijuana or controlled substance as evidenced by a stamp or other official indicia.”

Prosecutors estimate the tax on five pounds of marijuana is about $7,800. The penalty for violating the law is to pay the tax, serve not more than five years in prison, and pay up to a $10,000 fine.

Prosecutors said state police pulled over the suspects’ car at the Drum Hill Rotary in Chelmsford on Saturday for several reasons: The car appeared to have an altered exhaust system, a lowered suspension, excessive window tinting and was driving 10 mph under the speed limit. Prosecutors said troopers smelled what they believed to be marijuana, and found multiple vacuum-sealed bags containing a total of five pounds of pot. None of the bags carried tax stickers.

When the men were asked if they had paid the tax to the state Department of Revenue, Prum allegedly laughed and asked if he could apply for tax-exempt status, prosecutors said.

In Lowell District Court this week, defense attorney Glen Randall, who represents Prum, said he has never heard of such a law. Attorney Jamal Aruri, who represents Lakongpheng, said this was a “bad stop … The trooper stopped a car with minorities on a hunch that something was going on. I don’t see anything to justify this stop.”

Both men pleaded not guilty to charges of drug dealer failing to pay tax, possession with intent to distribute a class D drug, and equipment violation.

Lakongpheng was also charged with tinted-window obstruction.

Judge Elizabeth Cremens released both men on personal recognizance. A pretrial conference is scheduled for June 6.

Follow Lisa Redmond at www.twitter.com/lredmond13.