A series of competition experiments with two genotypes of Escherichia coli showed that each genotype was favored when it was the minority, allowing their stable coexistence. In these experiments, glucose was the sole source of carbon provided and its concentration was limiting to population density. Thus, the stable polymorphism does not conform to a simple model of competitive exclusion. We considered two hypotheses that might explain the observed coexistence: (1) a strictly demographic tradeoff, such that one genotype is competitively superior when glucose is abundant whereas the other genotype is the better competitor for sparse glucose; and (2) a cross-feeding interaction, whereby the superior competitor for glucose excretes a metabolite that acts as a second resource for which the other genotype is the better competitor. Although there was a demographic tradeoff, the advantage to the superior competitor at high glucose concentrations was too large (given the initial concentration of glucose used in these experiments) to allow the second genotype to invade when rare at the observed rate. Therefore, the second genotype must have some other advantage that allows it to readily invade a population of the superior competitor for glucose. Indeed, the second genotype could increase in abundance after glucose was depleted, but only in the presence of the superior competitor for glucose, thus implicating a cross-feeding interaction. These results confirm earlier studies showing that populations of E. coli can maintain ecologically relevant genetic diversity even in a simple environment.