Bishop Thomas J. Welsh, the controversial and conservative Roman Catholic bishop who has led the Diocese of Arlington since its creation 8 1/2 years ago, will be transferred to Allentown, Pa.

The transfer was announced yesterday by Bishop Pio Laghi, Pope John Paul II's apostolic delegate to the United States. No replacement for Welsh and no date for his installation in Allentown were announced. A spokeswoman for the Diocese of Arlington said Welsh did not request the transfer. "He was surprised," she said.

In a message to the diocese, Welsh said he will "find it quite difficult to leave Virginia." He called his time in the diocese "years of many graces" and said he "will not forget the support and encouragement you have given to me."

Speculation on the reasons for the transfer, which had been rumored in both Arlington and Allentown for months, ranged widely among priests and lay members of the diocese yesterday, with some viewing the move as a reward and others calling it a form of banishment because of confrontations Welsh had with Northern Virginia parishoners.

"I think it's a reward," said the Rev. Franklin McAfee, director of the family life bureau of the Diocese of Arlington. "It the Diocese of Allentown is a traditional type of diocese. People have substantial roots there." Priests in that diocese "wanted him," McAfee said.

Allentown Bishop Joseph McShea, who retired yesterday at age 75, also is a bishop who has sometimes been criticized as too conservative. Both McShea and Welsh were born in the Allentown diocese, and both served as auxiliary bishops in the Archdiocese of Philadelphia. Welsh is a former rector of Philadelphia's St. Charles Borromeo Seminary.

One Northern Virginia priest and Welsh ally speculated that the 61-year-old Welsh was returned to Pennsylvania to be groomed to replace conservative Philadelphia Cardinal John Krol, who must submit his resignation as the Archbishop of Philadelphia 2 1/2 years from now at age 75, as required by the church. Welsh was considered a Krol protege during his tenure in the archdiocese. Liberal Catholics who have been involved in feuds with Welsh disagreed. "I think the timing is very interesting in light of the problems we've had with the bishop," said Ruth Johnson, a leader of dissident church members at Holy Spirit parish in Annandale, where Welsh supported a priest who fired lay members of an elected parish council. That decision was appealed to the Vatican by church members.

"You just have to speculate that the Vatican didn't want him in Arlington any longer," said the Rev. Thomas Quinlan, a priest who left the Arlington diocese for the more liberal Richmond diocese after a confrontation with Welsh. "It's certainly not a promotion."

Welsh will move from a diocese near the nation's capital to the largely coal mining, industrial and rural area surrounding the Allentown-Bethlehem-Easton metropolitan area of about 500,000. The diocese has more Catholics and more priests than the Diocese of Arlington. The diocese has 263,365 Roman Catholics in five counties in central Eastern Pennsylvania--more than one-fourth the population of the area. The Arlington diocese's 15 counties hold 179,000 Catholics out of a total 1.4 million people.

Most of Welsh's disputes in Virginia took place in parishes in the Washington suburbs, which hold a sophisticated, highly educated population with money and power. Welsh became its first bishop when the diocese was carved out of the traditionally liberal Diocese of Richmond.

Welsh is known as an efficient administrator who focused on building his new diocese and for his antiabortion campaigning. He led marches on abortion clinics and lobbied against abortion on Capitol Hill.