Five plant checklists from areas on and adjacent to the Guiana Shield were analyzed and used to test hypotheses of the salient properties of the flora of northeastern South America. Newly available checklists of Iwokrama, Mabura Hill, and Kaieteur Falls (all three in Guyana), Central French Guiana, and Reserva Ducke near Manaus, Brazil were compiled and their nomenclatural synonymies standardized. Data from checklists provide an attractive alternative to transect or plot data because they are based on rigorously determined plants of all habit types rather than the sterile vouchers of tree species often used in plot or transect studies. Descriptive data were compiled from the checklists regarding diversity, overlap, and endemism. Ranking of diversity at the family level was subjected to Kendall's coefficient of concordance of ranks test and Spearman rank correlation coefficients to evaluate similarities among the five sites. A UPGMA dendrogram was created from data for the presence or absence of species shared by two or more sites. The results indicate strong similarity among the three sites in Guyana and between Central French Guiana and Reserva Ducke, supporting a model of plant distributions determined by the presence or absence of sandstone or white sands rather than disjunct between the Roraima sandstone formation (“Guiana Highland”) and other areas of the Guiana Shield. Relatively little overlap was found even between adjacent areas with ostensibly very similar abiotic environments, indicating that plant diversity will not necessarily be protected by conserving representative areas selected on the basis of general characteristics.