On February 21, 2007, Wanda Krupski tripped over a cable and fractured her leg while she was on board the cruise ship Costa Magica. The ticket identified the carrier as “Costa Crociere S. p. A., an Italian corporation.” It further provided that lawsuits had to be “filed within one year after the date of injury.” On February 1, 2008–three weeks before the one-year limitations period expired–Krupski filed a negligence action against Costa Cruise. The complaint alleged that Costa Cruise “owned, operated, managed, supervised and controlled” the ship on which Krupski had injured herself.

Over the next several months, after the limitations period had expired, Costa Cruise brought Costa Crociere’s existence to Krupski’s attention. Krupski ultimately amended the complaint under Federal Rules of Civil Procedure 15 to add Costa Crociere and then dismissed Costa Cruise. Shortly thereafter, Costa Crociere, represented by the same counsel who had represented Costa Cruise, moved to dismiss, contending that the amended complaint did not relate back under Rule 15(c) and was therefore untimely.

The district court granted the motion. The 11th Circuit affirmed. Because the cruise ticket clearly identified Costa Crociere as the carrier, the court stated, Krupski either knew or should have known of Costa Crociere’s identity as a potential party. It was therefore appropriate to treat Krupski as having chosen to sue one potential party over another, rather than having made a mistake.

The Supreme Court reversed. Krupski v. Costa Crociere. One of the requirements for an amended complaint against a newly named defendant to related back to the original complaint is that, within the period set forth in Rule 4(m) (normally 120 days from when the complaint is filed), the newly named defendant “knew or should have known that the action would have been brought against it, but for a mistake concerning the proper party’s identity.” Rule 15(c)(1)(C)(ii).

Rule 15 asks what the prospective defendant knew or should have known during the Rule 4(m) period, not what the plaintiff knew or should have known at the time of filing her original complaint. Or, as applied in this case, the question under Rule 15 was not whether Krupski knew or should have known the identity of Costa Crociere as the proper defendant, but whether Costa Crociere knew or should have known that it would have been named as a defendant but for an error.

Under these principles, the courts below erred in denying relation back. Because the original complaint (of which Costa Crociere had constructive notice) made clear that Krupski meant to sue the company that “owned, operated, managed, supervised and controlled” the ship on which she was injured and also indicated (mistakenly) that Costa Cruise performed those roles, Costa Crociere should have known that it avoided suit within the limitations period only because of Krupski’s misunderstanding about which “Costa” entity was in charge of the ship–clearly a “mistake concerning the proper party’s identity.”

The 11th Circuit also erred in ruling that Krupski’s undue delay in seeking to file, and in eventually filing, an amended complaint justified its denial of relation back under Rule 15(c)(1)(C). As the Supreme Court explained, Rule 15(c) plainly sets forth an exclusive list of requirements for relation back, and the plaintiff’s diligence is not among them.