Background/Question/Methods Invasive plants pose a serious threat to native biodiversity and the ability of these species to invade a habitat may be facilitated by other anthropogenically-driven environmental change factors, in particular soil nitrogen enrichment. Numerous theoretical and empirical studies indicate that non-native plants are most likely to invade environments where resource availability exceeds resource demand by resident species (e.g. nitrogen is not limiting). Furthermore, invasive species have been shown to demonstrate greater phenotypic plasticity in a variety of traits when compared to co-occurring native species and thus may respond more strongly to increased soil nitrogen availability. Also, invasive plants may experience less population regulation compared to native species if their specialist enemies (e.g. herbivores and pathogens) are not present in the introduced range. The purpose of the current study was to investigate factors that may contribute to the invasiveness of an introduced plant species, Dalmatian Toadflax, Linaria dalmatica (L.) P. Mill. (Scrophulariaceae). In particular, this project was focused on examining the influence of soil nitrogen availability and the presence of insect biological control agents on plant defensive chemistry in four populations of Dalmatian Toadflax located in Boulder County, Colorado, USA. Results/Conclusions Results indicate significant differences in plant defense compounds among the four populations (p < 0.001); the population with the lowest levels of defense compounds was the only population in which insect biological control agents were not present. Additionally, the influence of soil nitrogen availability on plant defensive chemistry varied among populations. There was a highly significant negative relationship between soil nitrogen availability and plant defensive compounds in one population (P=0.002,R2=0.53), and a marginally significant negative relationship in a second population (P=0.068, R2=0.27). However, there was no significant relationship in the remaining two populations. The results of this study indicate that Dalmatian Toadflax demonstrates phenotypic plasticity in defense compounds in response to biotic and abiotic factors. Further, results suggest that in high nitrogen environments plant resources may be allocated to growth rather than defense.