Gracilaria salicornia and G. parvispora were introduced to the south reef of Moloka‘i, Hawai‘i, in the past 15–20 yr for aquaculture development. Both species have naturalized on the reef. Gracilaria salicornia is now considered an invasive species on O‘ahu due to its tendency to grow in dense beds that produce undesirable windrows of thalli on the beach. There is also concern that it reduces biodiversity and degrades habitats of reefs. We surveyed the south coast of Moloka‘i, where both species were introduced, and measured biomass density, growth rates, and thallus nutrient contents of G. salicornia in established beds. Both species are found in the silt-laden, nearshore zone of the reef within 50 m of shore. Gracilaria salicornia grows in dense beds containing 475 g dry weight m-2 of biomass, but growth rates are low, 0.03%–1.28% day-1. Tissue nitrogen levels are low, suggesting that these populations are nitrogen limited. Nevertheless, populations of G. salicornia persist and grow slowly on the reef, whereas those of G. parvsipora are only found in areas of local nitrogen enrichment from anthropogenic sources. Currently, G. salicornia does not appear to be negatively affecting the reef ecology on Moloka‘i, because it is confined to the disturbed, nearshore zone. However, its ability to grow slowly and persist under low-nitrogen conditions allows it to form dense beds and suggests that it will eventually spread farther along the coast.