Another Part of the Forest. Not quite up to the level of Lillian Hellman's The Little Foxes, although it contains many of the same characters. Still worth seeing, though. At the Loeb tonight and tomorrow and July 21-26 at 8, except Saturdays at 5 and 9.

Lobsterback. Apparently a euphemism for redcoat. Yes, you guessed it, another Bicentennial play, but with a new twist--this one was written by British dramatist James Forsyth, and appropriately enough concerns the difficulties faced by a young British soldier and the Milton family that befriends him. Like Ryan's Yorktown Tune, this play was especially commissioned by Tufts University in honor of the Bicentennial, and it promises to be as good as the previous production, if not better. At the Jufts Arena Theater in Medford at 8:15 tonight and tomorrow, July 22-26 and July 29-August 2. Tickets are $4 Fridays and Saturdays, $3 other nights.

Sir and William Penn. Another original Bicentennial play, but this one never even gets to America. It's about William Penn's problems in England, where he underwent a trial for his Quakerism. In the end, his father gives him Pennsylvania, where he will go to find religious freedom and live happily ever after. At Old West Church, 131 Cambridge St.in Boston. Tonight and Thurs., Fri., and Sat.nights through August at 8 p.m. Reservations necessary, 227-8020.