This paper describes the alignment system for verbal person-marking in Chitimacha, a language isolate of Louisiana. Using data from recently digitized versions of texts collected by Morris Swadesh in the 1930s, I show that Chitimacha exhibits a split alignment system with agent-patient alignment in the first person and nominative-accusative alignment in non-first persons. The agent-patient ... [Show full abstract] alternation is shown to cross-cut subjects of intransitives, objects and even subjects of transitives, and direct/indirect objects of di-transitives. The agent-patient system in Chitimacha is therefore sensitive not to transitivity but rather to the semantic categories of agent and patient, making it an exemplary case of semantic alignment. I also discuss evidence of the diachronic origins of the agent-patient pattern and show that it arose via a reanalysis of transitive verbs with impersonal subjects ("transimpersonals") as intransitive verbs with patientive subjects.