The allelopathic compounds secreted into the soil by Brassica nigra (L.) W. D. J. Koch (black mustard) serve to reduce seed germination rates and plant fitness in nearby species. As such, B. nigra shares in an amensalistic relationship with most of the species within range of its allelopathic interference. However, Cucurbita foetidissima (buffalo gourd) is a native species in southern California that has managed to persist within and around areas invaded by B. nigra. We hypothesized that increased proximity to B. nigra would negatively impact C. foetidissima fitness and alter life history strategies. We found that C. foetidissima growing in close proximity to B. nigra had smaller fruit volume, a lower number of seeds per fruit, lower dry seed weight, a higher number of seeds per cubic centimeter, and a higher number of seeds per gram of fruit than C. cucurbita that grew outside of the allelopathic range of B. nigra. The reproductive strategy of C. cucurbita also appeared to change from K-selection to r-selection in the presence of allelochemicals.