PORTLAND – The founder of Idexx has named a successor and is stepping aside – for the second time in three years.

David Shaw is handing off the titles of chairman and chief executive officer to Jonathan W. Ayers and resigning from the board.

Shaw, 50, started the Portland veterinary biotech firm in 1984. Since then, Idexx grew to a company with more than 2,000 employees and revenues of nearly $100 million in the last quarter.

But the company’s growth ground to a halt in 1997, when the company told investors that sales would be much lower as it sought to reduce inventory. The stock price slid from $32 a share to $12 in one day.

The company since has recovered and Shaw says it’s time to let someone else manage Idexx on a day-to-day basis.

In 1999, Shaw promoted Jeffrey Langan from chief operating officer to chief executive officer while Shaw retained the title of chairman. But Langan was gone in six months and Shaw was back in charge.

This time around, Shaw is dropping the title of chairman and leaving the board of directors.

“I do feel that when you put someone in charge that they be put fully in charge and there should be no question about that,” Shaw said.

Shaw said he will continue to have a significant ownership share in Idexx.

As of last April’s proxy statement, Shaw owned more than 1.4 million shares, or 4.15 percent of the total. His stock was worth more than $34 million at Thursday’s closing price of $24.28.

Ayers, 45, is the former president of United Technologies Corp.’s Carrier unit and worked in the parent company’s corporate offices as vice president of strategic planning before that.

Ayers previously worked on mergers and acquisitions for Morgan Stanley and was a manager for Bain & Co., a Boston consulting firm.

Shaw said the prospects for Idexx are good. A new drug and blood analysis machine are both on track to be approved for sale this year, he said.