Kroger Chairman and CEO Rodney McMullen has resigned after an internal investigation into his personal conduct.

One of the largest grocery chains in the country said Monday that the investigation into McMullen's personal conduct was unrelated to the business, but was found to be inconsistent with its business ethics policy.

Board member Ronald Sargent will serve as chairman and interim CEO, effective immediately.

Sargent has been on Kroger's board since 2006 and has served as the lead director of the company since 2017. He's worked in several roles at the grocery chain across stores, sales, marketing, manufacturing and strategy. Sargent is also the former Chairman and CEO of Staples.

Kroger said its board was made aware of the situation on Feb. 21 and immediately hired an outside independent counsel to conduct an investigation, overseen by a special board committee.

The company said that McMullen's conduct is not related to its financial performance, operations or reporting, and did not involve any Kroger associates.

Kroger will conduct a search for its next CEO, with Sargent agreeing to remain as interim CEO until someone is appointed to the role permanently.

Shares of Kroger, based in Cincinnati, fell about 1.3% before the opening bell.