Family Dollar Stores adopted a one-year poison pill with a trigger of 10 percent, two days after activist investor Carl Icahn reported a stake in the company and said he would consider pushing for a merger with Dollar General.
Icahn reported a 9.39 percent stake in the company on Friday, making him its largest shareholder.
Read MoreIcahn takes 9% stakein Family Dollar
The rights plan is not designed to prevent an offer to acquire the company but to allow its board "adequate time to consider any and all alternatives," Family Dollar said on Monday.
"Poison pills" are designed to stop hostile takeover attempts by triggering the issue of new shares that dilute the holdings of investors who exceed a set threshold.