To complete their maturation, most immature thymocytes depend on the simultaneous engagement of their antigen receptor [alpha beta T cell receptor (TCR)] and their CD4 or CD8 coreceptors with major histocompatibility complex class II or I ligands, respectively. However, a normal subset of mature alpha beta TCR(+) thymocytes did not follow these rules. These thymocytes expressed NK1.1 and a restricted set of alpha beta TCRs that are intrinsically class I-reactive because their positive selection was class I-dependent but CD8-independent. These cells were CD4(+) and CD4(-)8(-) but never CD8(+), because the presence of CD8 caused negative selection. Thus, neither CD4 nor CD8 contributes signals that direct their maturation into the CD4(+) and CD4(-)8(-) lineages.