Mouse T cells co‐expressing an αβ T cell receptor (TCR) and the NK1.1 antigen have been shown to be major interleukin (IL)‐4‐producing cells and could therefore regulate cell‐mediated immune responses. We have identified a related subset of thymocytes co‐expressing a γδ TCR and NK1.1 which also produce IL‐4. Unlike αβ⁺NK1.1⁺ thymocytes, the selection of γδ⁺NK1.1⁺ thymocytes is not dependent upon β2‐microglobulin (β2m)‐associated class I molecule expression because these cells are present in β2m‐deficient mice. This suggests that γδ⁺NK1.1⁺ T cells may regulate immune responses to a different variety of antigens. However, the development of αβ⁺NK1.1⁺ and αβ⁺NK1.1⁺ thymocytes appears to be related. Analysis of different mutant mice lacking αβ⁺NK1.1⁺ thymocytes revealed a specific increase in γδ⁺NK1.1⁺ thymocyte production when the block in αβ⁺NK1.1⁺ thymocyte differentiation occurs after β TCR rearrangement.