The concept that IL-4 is the primary signal for Th2 lymphocyte differentiation has recently been put in doubt by studies in which the production of Th2-associated cytokines was detected in mice deficient in IL-4 synthesis or IL-4R triggering. In this study, we formally demonstrate by single cell analysis that CD4+ lymphocytes with a classical Th2 phenotype (IL-4+, IL-5+, IFN-gamma-, IL-2-) develop in significant numbers in helminth-infected mice deficient in either IL-4R alpha-chain or Stat6. While an expanded population of Th1 (IL-4-, IL-5-, IFN-gamma+, IL-2+) lymphocytes was observed in the same animals, surprisingly, cells with a mixed Th0 cytokine pattern were rare. The cytokine production phenotypes of the Th1 and Th2 subpopulations generated in infected Stat6-deficient mice were unaffected by in vitro neutralization of endogenous IL-4 or IFN-gamma. Nevertheless, while addition of exogenous rIL-12 resulted in transitory IFN-gamma production by Th2 lymphocytes from both wild-type and Stat6-deficient mice, IL-4 synthesis was preserved in the former, but temporarily ablated in the latter cells. Importantly, IL-4+ IFN-gamma- and IL-4- IFN-gamma+ populations similar to those arising in helminth-infected Stat6-deficient mice could also be generated in vitro by repetitive polyclonal stimulation of CD4+CD62Lhigh lymphocytes from uninfected mice of the same strain. Together, the results of these single cell analysis experiments demonstrate that IL-4R/Stat6 signaling, while influencing the final frequency of Th2 lymphocytes, is not essential for Th2 cell development, and suggest that this pathway has a previously unrecognized function in stabilizing Th2 populations once they have emerged.