The responses of the solitary urochordate, Styela plicata, to first- and second-set tunic grafts confirm the existence of a sensitive histocompatibility system. Most first-set allografts were eliminated (median rejection time, RT50, of 38.2 ± 5.9 days) whereas the majority of autografts remained viable. Allograft rejection exhibited significant memory with second-set allografts being lost far more rapidly (RT50 = 22.0 ± 2.6 days) than first-set allografts. This alloimmune memory was shown to survive for up to 50 days after first-set rejection. Furthermore, 3rd party grafting indicated that memory was specific to the presensitizing tissue type. These results are discussed in terms of the evolution of vertebrate immunity.