Histoincompatibility was analyzed to describe as precisely as possible the sequence of gross morphological events taking place during allograft rejection among closely related conspecifics from a natural population of the bisexual lizard Cnemidophorus tigris. Two types of rejection were noted, depending on whether the grafts were sloughed or not. Abrupt rejection was typically characterized by hemorrhaging of the graft site, scale dissolution, and eventual sloughing of the graft. This process occurred in a graded sequence. Acute rejection occurred from 15 to 20 days, subacute from 30 to 45, and subchronic from 60 to 90. In the gradual form of rejection, the allografts were gradually replaced by host tissue during a period ranging from approximately 100 to 350 days post-transplantation. Such a gradation observed in both the abrupt and gradual categories suggests that large numbers of genes and/or alleles are responsible for the antigenic properties of skin in these lizards.