Abstract: The bounded hybrid superiority and tension zone models of hybrid zone structure differ in their assumptions regarding hybrid fitness.  This study addresses the relative fitness of a mint hybrid to its parental species by focusing on the effects of light, an environmental variable thought to be important in the system.  One parental species, Monarda clinopodia, is generally found in the understory, while the other, M. fistulosa, is generally found in sunnier areas.    Parentals and hybrids were collected from the field and grown for five weeks in one of three randomly assigned treatments- 80% shade, 50% shade, or full sun (control).  Fitness was assessed by measuring proportional changes in stem length, number of leaves, and biomass.  Data for stem length and leaves revealed no significant treatment effects; however, there is evidence that morphological differences among the taxa confounded the analysis.  Biomass data revealed  a significant treatment effect and interaction, with the hybrids performing better in the intermediate environment than in the full sun.  This result is consistent with the bounded hybrid superiority model; however, consistent with the tension zone model, there was no evidence for local adaptation in the parental species.