In an surprise announcement at last night's City Council meeting, Councillor Kathleen L. Born told her colleagues that she will not run for reelection this fall.

"Tonight I am announcing that I will not seek reelection," said Born, adding that having a blank slate before her as she approaches her 55th birthday and her 35th anniversary is "exhilarating."

"This is a sad day for the City Council," said Mayor Anthony D. Galluccio immediately after the announcement. "I'm not looking forward to coming into these chambers when you're not here," he said to Born.

Born, an MIT graduate and an urban planner by trade, frequently brought open space, zoning and affordable housing issues before the council.

Since she was elected in 1993 by a mere 12 votes, she has finished solidly in council elections in 1995, 1997 and 1999, and last year made a bid for the mayor's seat which fell just short.

With her impending retirement from the council, the Cambridge Civic Association (CCA) will suffer another major blow in its leadership.

One of Cambridge's two longtime political parties, the CCA traditionally fielded a slew of candidates in city elections, usually electing four or five to the City Council.

But after a series of divisive mayoral elections in the 1990s--in which both Reeves and former Councillor Katherine Triantafillou left the party--and first-term Councillor Marjorie C. Decker's refusal to join the CCA during the 1999 election, the CCA found itself with only three members on the current council.