Nothofagus spp. dominate the upper canopy of some rainforests on ultramafic soils in New Caledonia. These monodominant forests typically occur within, or contiguous with, larger areas of mixed-canopy rainforest. In this study the structure, diversity and composition of six Nothofagus-dominated plots were investigated, and comparisons were made with three adjacent mixed rainforest plots. Stand density and basal area (all stems ? 1.3 m high) in the Nothofagus plots were in the range 16,056-27,550 stems/ha and 43.1-69.9 m2/ha, respectively. There was no significant difference (P ? 0.05) in total stand density or basal area between the paired Nothofagus and mixed rainforests, but there were consistently fewer trees and less basal area of trees ? 40 cm d.b.h. in the Nothofagus forests. Species richness, species diversity (Shannon-Wiener, based on basal area) and equitability (based on basal area) of trees ? 20 cm d.b.h. on 0.1 ha Nothofagus plots were in the range 4-17, 0.96-3.76 and 0.45-0.87, respectively. No significant differences (P ? 0.05) were recorded in these three parameters between the paired Nothofagus and mixed rainforests, although species diversity was consistently lower in the paired Nothofagus forests. Comparison of dominance by density and basal area indicated that although the uppermost canopy of the Nothofagus forests was dominated by Nothofagus (70-95%), the basal area and density contribution was ? 55% except at Col de Yate (? 85%). Analysis of similarity indicated no significant difference in stand composition of trees ? 20 cm d.b.h. (following removal of Nothofagus from the data set) between Nothofagus and mixed rainforests using basal area, density or presence-absence data. It is concluded that the Nothofagus-dominated forests differ from the adjacent mixed rainforests mainly by (1) dominance of the uppermost canopy, without necessarily dominance of the stand by basal area or density, and (2) the smaller basal area contributed by large trees (all species).