HARRISBURG, Pa. (AP) _ A state senator who was convicted of polluting a stream while responsible for a sewage-treatment plant resigned Monday, less than an hour before an ethics panel was to consider his status in the Legislature.

Sen. William L. Slocum, a first-term Republican, was sentenced last week to a month in prison and five months of home detention for polluting a northwestern Pennsylvania stream during the 12 years he oversaw the Youngsville sewage-treatment plant.

Slocum, 52, is the fourth member of the General Assembly to resign since February. His resignation takes effect June 1.

``Serving the people of this great commonwealth as a senator has been the greatest honor and privilege of my life,'' Slocum wrote in a three-paragraph letter to Lt. Gov. Mark S. Schweiker, who presides over the Senate.

Slocum pleaded guilty to six federal misdemeanor charges of negligent discharge as part of an agreement with prosecutors. Prosecutors said he allowed raw sewage to flow into Brokenstraw Creek in Youngsville on several occasions between 1983 and 1995, while he worked for the Youngsville water department and as borough manager.

Democrats called for Slocum's resignation immediately after the sentencing. Gov. Tom Ridge, a Republican, said the lawmaker should consider quitting. A Senate ethics panel had been scheduled Monday to discuss whether to expel or censure Slocum over the charges.

A woman who answered the telephone at Slocum's home said the lawmaker would not comment.

The three other state lawmakers who resigned this year were all in the House: Republican Tracy Seyfert pleaded guilty to illegally obtaining a generator intended for firefighters; Democrat Frank J. Gigliotti was charged with receiving bribes; and Republican Frank A. Serafini was sentenced on federal perjury charges.

Another state representative, Republican Thomas W. Druce, is awaiting trial on vehicular homicide in a fatal hit-and-run case.