Most marijuana users who get caught smoking a joint summarily pay a fine, but when an undercover police officer detained Richard E. Cusick and R. Keith Stroup, the two chose instead to challenge the constitutionality of Massachusetts laws banning marijuana for the first time in 30 years.

Arrested for sharing a marijuana cigarette at the annual Boston Freedom Rally in September, Cusick and Stroup turned to Harvard Law School professor Charles R. Nesson ’60 for legal counsel. Nesson and his clients acknowledged that they had used the illegal drug, and decided upon an unusual defense: they argued that the statute outlawing marijuana in Massachusetts has no “rational basis,” and that the jury has the power of jury nullification, or ruling a defendant innocent while recognizing that he or she had violated a law.

Before the trial, which began Tuesday, Nesson called the case “extraordinary,” adding that it offered the chance to address larger legal issues, such as the meaning of a crime.

“On the one hand, it’s asserted that a crime is a violation of a statute, which has a certain circular quality to it,” Nesson said. “The contrasting view is that a crime is an offense against society, a behavior that offends.”

Both co-defendants built their careers around marijuana: Cusick is associate publisher of the well-known marijuana magazine, High Times, and Stroup is the founder of the National Organization for the Reform of Marijuana Laws. They said that they do not believe marijuana to be a social ill.

Nesson said he had hoped that several experts—including Lester S. Grinspoon, an associate professor emeritus at Harvard Medical School, and Jeffrey A. Miron, director of undergraduate studies in economics—would be allowed to testify to the harmlessness of marijuana.

But the defendants were not granted an evidentiary hearing, and the jury found them guilty of marijuana possesion after deliberating briefly. The judge sentenced them to one day in prison, which they had already served the day they were arrested.

“The idea that they were found guilty of a crime was just crushing to me,” Nesson said in an interview yesterday.

Although Nesson plans to file an appeal, it is unlikely that an appellate court will rule to change the verdict.

Stroup also said that he was disappointed with the outcome, but he praised the fairness of the trial.

“I can’t feel too bad that we were able to be honest about our marijuana smoking, to openly challenge the marijuana law in Massachusetts, and to be treated fairly and leniently,” he said. “We’ve come a good distance in this country.”

—Staff writer Athena Y. Jiang can be reached at ajiang@fas.harvard.edu.