AbstractNui Atoll, an isolated atoll in Tuvalu, has a total vascular flora of 86 species, including a large number of introduced species. If only the 44 species considered indigenous are examined, there exists a strong linear relationship (r2= 0.94) between the total number of species and the logarithm of island area, over the entire wide range of island sizes from 0.01 ha to 138 ha. This relationship is strong for both three species and herb species, but weak for shrub species for which there is a limited pool of potential colonists. This relationship is stronger than on other Pacific atolls and does not support the existence of a ‘small island effect’. The strong linear relationship on Nui is attributed to uniformity of reef island substrate, distribution and isodimensionality of reef islands, stability of islands and infrequency of catastrophic events and the absence of large monospecific stands, especially of shrub species, characteristic of drier ato