Little is known about the demographic consequences of fragmentation for plant populations. By assessing the population structure of a common herb ( Heliconia acuminata) in an experimentally fragmented landscape in the central Amazon, we tested the predictions that fragmentation could reduce population density, alter population structure, and reduce reproductive effort. Population density in continuous forest varied six-fold, some areas having high density and others low density. Population density in small fragments and on the edges of large fragments was often low, but it was within the range of densities found in continuous forest, and the difference between locations was not significant. Heliconia populations in forest fragments were skewed toward smaller demographic size classes, however. Because reproduction in H. acuminata is positively correlated with plant size, these shifts were predicted to result in fewer flowering plants in forest fragments. The proportion of the population flowering in forest fragments and continuous forest was not significantly different, although there was a trend toward proportionately greater flowering in continuous forest. For plants that did flower, per-individual reproductive success (measured as developing fruit set) was the same in forest fragments and continuous forests. Our results suggest that per-individual and population-level reproduction by understory herbs in tropical forest fragments may be resistant to the detrimental consequences of fragment isolation. Our study also highlights the need to consider how fragmentation influences aspects of population structure and demography beyond abundance and fruit production, because these alternative measures of population structure can be modified in forest fragments in subtle, unexpected ways.