CEO Pamela D.A. Reeve stepped down Monday.
The company named director Robert Donahue interim CEO and said it will search for a full-time replacement.
The news came at the end of a day in which shares of the Burlington, Mass., provider of customer-transaction management technology solutions dropped 5%.
"I have enjoyed my nearly 15 years as President and CEO of Lightbridge and the relationships I have forged with management and the Company's clients, partners, employees and stakeholders," Reeve said. "After many years of leading Lightbridge, I believe this is a good time to transition from the Company to pursue personal and business opportunities that I have been interested in for some time."
"We wish her well as she ventures off into this new phase of her life," Chairman Kevin Melia said.
Donahue is a member of the Lightbridge board and has been chairman of the audit committee since January.
Lightbridge shares dropped 20 cents Monday to $4.10.