The expression of IFN-γ is a hallmark of Th1 cells and CD8+ effector T cells and is the signature cytokine of type 1 responses. However, it is not known whether T cells are homogeneous in their capacity to produce IFN-γ, whether this potential varies between tissues, and how it relates to the production of other effector molecules. In the present study we used bicistronic IFN-γ-enhanced yellow fluorescent protein (IFN-γ-eYFP) reporter mice (Yeti) and MHC class I tetramers to directly quantify IFN-γ expression at the single cell level. The eYFP fluorescence of Th1 cells and CD8+ effector T cells was broadly heterogeneous even before cell division and correlated with both the abundance of IFN-γ transcripts and the secretion of IFN-γ upon stimulation. CD4+ and CD8+ T cells of influenza-infected mice revealed a similarly heterogeneous IFN-γ expression, and eYFP high cells were only found in the infected lung. Ag-specific T cells were in all examined tissues eYFP+, but also heterogeneous in their reporter fluorescence, and eYFP high cells were also restricted to the infected lung. A similar heterogeneity was observed in Toxoplasma gondii-infected animals, but eYFP high cells were restricted to different tissues. Highly eYFP fluorescent cells produced elevated levels of proinflammatory cytokines and chemokines in addition to IFN-γ, suggesting their coregulated expression as a functional unit in highly differentiated effector T cells.