She begins the newly created position on Thursday and will work with co-director Thomas H. Luster, vice president, who had been sole director of the team.
Ms. Gaffney was co-portfolio manager of the Loomis Sayles Bond Fund with Daniel Fuss, vice chairman and portfolio manager.
Ms. Gaffney joined Mr. Fuss on the management team of the $22 billion fund in 1997 and was widely seen as the legendary bond manager's eventual successor. Under the pair's guidance, the Loomis Sayles Bond Fund had an annualized return of 8%, more than 200 basis points better than that of the Barclays Capital Aggregate Bond index.
Ms. Gaffney also was portfolio manager on numerous other mutual funds, including the $1 billion Loomis Sayles Fixed Income Fund. All told, she was managing more than $73 billion in fixed-income assets for Loomis Sayles.
Mr. Fuss will manage the Loomis Sayles Bond Fund with Matthew Eagan and Elaine Stokes, who were added as managers in 2007.
Loomis Sayles spokeswoman Erin Heard said there are no plans to replace Ms. Gaffney.
Jason Kephart, a reporter with InvestmentNews, a sister publication of Pensions & Investments, contributed to this story.