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We investigated the potential role of a nitrogen-fixing legume in facilitating invasion by an alien grass, Pennisetum setaceum (fountain grass) into native Heteropogon contortus grasslands in Hawai‘i. Both grasses are C4 perennials. Many formerly extensive Heteropogon grasslands have been reduced to remnants surrounded by Pennisetum. An inconspicuous annual legume, Chamaecrista nictitans (partridge pea), was commonly associated with Pennisetum stands but was found at higher densities in Pennisetum-Heteropogon mixtures. In the field, we also found higher exchangeable soil nitrogen beneath Chamaecrista-Pennisetum-Heteropogon mixtures than beneath neighboring pure Heteropogon stands. We then used greenhouse experiments to test the hypothesis that increased nitrogen facilitates Pennisetum's invasion of established Heteropogon. Under low nitrogen conditions, Pennisetum seedlings planted beneath established Heteropogon plants grew very little (<0.3 g biomass after 16 weeks), and their growth was not affected by high or low water treatments. In contrast, under high nitrogen conditions, in both low and high water treatments, Pennisetum seedlings grew rapidly, averaging 60 fold the biomass of seedlings in the low nitrogen treatment and exceeding the biomass of the established Heteropogon plants under which they were planted. We also compared Pennisetum-Heteropogon seedling competition in the presence and absence of live Chamaecrista. The growth of Pennisetum was not directly facilitated by live Chamaecrista in the greenhouse pots, but the Chamaecrista did not increase available soil nitrogen until after it had died. Our results suggest that increases in soil nitrogen, like those associated with an inconspicuous alien legume, can facilitate an alien grass’ invasion of a native grassland.