Garlic mustard (Alliaria petiolata, Brassicaceae) is a biennial herb that produces glucosinolates, a class of constituent secondary metabolites that defend against herbivores and pathogens allowing it to grow at high densities in invaded regions. The glucosinolate sinigrin is predominant in garlic mustard and aids in its competitiveness as an invasive species. In North America, garlic mustard can grow at high densities and form dense monocultures which may increase its apparency to herbivores and therefore increase its sinigrin production. I measured leaf sinigrin concentration in garlic mustard populations of different densities in the field and in greenhouse experiments to evaluate the response of sinigrin concentration and growth to density and light. Sinigrin concentrations of second-year plants were negatively correlated with growth metrics across all field densities; indicating a cost to sinigrin production. In the greenhouse density experiment with high and low rosette stem densities, sinigrin differed significantly by rosette density category. A factorial greenhouse experiment with light and density treatments discerned significant differences in sinigrin concentration by density. These findings suggest that sinigrin concentration may be influenced by intraspecific density across different light environments.