A New York law school professor who works in his Connecticut home part of the week can be taxed by both states, a New York court has ruled, heightening the debate over an issue important to the increasing number of Americans who use their home computers to telecommute.

The Nov. 2 ruling by the New York State Division of Tax Appeals was one of several recent cases in which courts have wrestled with different state income tax standards. Some states, including Connecticut, base income tax on residency. Others, such as New York, base their tax on where money is earned.

The question arising with increasing frequency is this: When a resident qualifies for taxation by more than one state, which is entitled to collect? In the New York case, both states won. Professor Edward A. Zelinsky of Yeshiva University Benjamin N. Cardozo School of Law lost.

Prof. Zelinsky and his wife face double taxation on the portion of his salary that is earned from home. The ruling upholds the constitutionality of the "convenience of the employer test," which is embodied in New York tax law.

Prof. Zelinsky, who appeared pro se, contends his case is one of several in which the employer convenience doctrine is being questioned in light of modern technological reality.

"The issue is one of increasing importance in a world where you have more . . . telecommuting. There is also a very serious conflict between the states," Mr. Zelinsky said, adding that he will appeal.

The ruling arose from a challenge by Mr. Zelinsky and his wife, Doris. A resident of New Haven, Conn., and a tax professor at Cardozo, he commutes to New York a few days a week. On other days, he works from home. Because he is employed by a university in New York, state officials there claimed the right to tax his entire salary. However, Connecticut made the same claim because the state bases its income tax on residence.

After New York denied the couple's petition for a refund, they challenged the constitutionality of the employer convenience doctrine. The doctrine states that income earned by a nonresident employee who chooses to work at home for convenience is deemed New York-source income subject to New York tax. The rule was established under the theory that because a New York resident would not receive any tax benefit for working from home, neither should a nonresident.

Additionally, courts upholding the rule have expressed concern over the potential for abuse if apportionment of tax liability is predicated solely on the physical presence of the employee.

Mr. Zelinsky did not dispute the constitutionality of the doctrine on its face. Rather, he argued that under the facts of his case, in which Connecticut employs a residency test and does not recognize New York's convenience-of-the-employer approach, the application of the New York doctrine violates the due process and commerce clauses of the Constitution. He argued that income earned while working in Connecticut should be apportioned to Connecticut.

The court, however, said that there is a nexus between his income and New York, and that therefore "it cannot be said that the income in question bears no reasonable connection or fair relationship to New York such that subjecting the same to New York income tax violates due process."

The court also rejected Mr. Zelinsky's commerce clause argument. "Simply because Connecticut does not . . . recognize the income in question as New York source income, and instead taxes the same as Connecticut source income without credit for taxes paid to New York, does not serve to invalidate the regulation in issue as applied to petitioner or require New York to disavow the income as New York source income properly subject to tax," the court wrote.

The decision has troubling implications, said Walter Hellerstein, a tax expert and law professor at the University of Georgia at Athens, who has written extensively on local and state taxation.

"Suppose I were a professor at New York University and was living in Paris and arranged to do classes by video, without New York giving me any protection or benefit. . . . Can New York tax me on that income?" Mr. Hellerstein asked.

Normally, he said, a state that bases tax on residence gives a credit for a tax against a source state, but only for income derived from that state. However, Connecticut does not recognize these earnings as New York-source income. The court said that both New York and Connecticut have valid sourcing rules, subjecting Mr. Zelinsky to taxes by both.