ABSTRACT The densities of the breeding populations and the sex of all flowering individuals were recorded for five dioecious canopy tree species of Central Amazonian Myristicaceae, in 11 study areas of the Minimum Critical size of Ecosystems Project totalling 22.5 ha. Adult population densities were extremely low, ranging from 0.38 to 1.61 ha –1 for the five species studied. In a 10 ha study plot the mean distance to the nearest flowering conspecific ranged from 48 to 100 m, while the mean distance to the nearest opposite sex conspecific was 147 m. The two most abundant species, Iryanthera macrophylla and Virola calophylla , both showed male-biased sex ratios, of 23:9 and 20:6, respectively. The size class distribution of males, females and non-flowering individuals in V. calophylla suggests that earlier reproductive maturation of male plants may provide a partial explanation for this bias. In I. macrophylla , since 95% of the individuals were observed flowering, the observed ratio is representative of the population, and may be caused by sex shifts from male to female. The low reproductive densities, combined with the skewed sex ratios and overlapping generations of these species, create very small effective breeding populations, placing species such as these at great risk in the face of deforestation and habitat fragmentation.