Nine skulls of the rare Oryzomys argentatus are compared to 109 skulls of the six races of O. palustris. Mahalanobis distance is greater between O. argentatus and all Floridian forms of O. palustris than the Floridian forms are from each other. In a canonical discriminant analysis, two models grouping O. argentatus with one or both of the insular races of O. palustris (sanibeli and planirostris) were shown by the Roy's Greatest Root statistic to fit the data less well than a model in which O. argentatus was regarded as distinct. A one-way ANOVA and Duncan's Multiple Range Test on the variation in nasal bone proportions show that there are two significantly different groups of these Oryzomys (p < 0.05): all O. palustris together and O. argentatus alone. We hypothesize O. argentatus originated on the Lower Keys in the late Sangamon and underwent selection for character divergence in sympatry with O. palustris during the Wurm.