Paul Hill Jr. resigned yesterday as chairman and chief executive officer of the troubled Chemical Safety and Hazard Investigation Board, citing "costs to my personal life" that made it impossible to continue.

The resignation ended a bitter power struggle between Hill and the other three board members over their respective roles in policy and administration and could begin a retooling of the crippled agency.

After a board meeting, Hill issued a statement announcing his resignation as CEO and chairman, positions he has held since the agency began work last year. Hill said he will continue as a board member.

President Clinton could appoint one of the other sitting members to replace Hill, or could name an outsider because the five-member board has a vacancy. The White House's intentions were not known yesterday.

Hill's statement made no specific mention of intramural conflict but said, "I came to realize that although the rewards of this position are great, the costs to my personal life are even greater. That price has now exceeded my personal drive to continue."

The board, patterned after the National Transportation Safety Board, has a mandate to investigate serious chemical industry accidents. It has completed three reports in the year it has been active.

Trouble began early this year when the newer board members--full-time experts appointed by the president--noticed that Hill and Chief Operating Officer Phyllis Thompson were making all agency decisions related to staff, funding and administration.

With neither Hill nor the rest of the board willing to yield, the agency had virtually ground to a halt by mid-year, but Hill's position was seriously eroded this month after Sen. Frank R. Lautenberg (D-N.J.), the board's biggest congressional booster, issued a statement favoring the board's position on power-sharing.

Lautenberg yesterday praised Hill's "vision and energy" but said his resignation suggested that the board "is resolving its governance problems." The agency still has "some distance to go," he continued, "but today a very difficult and important step was taken."