Allozymic comparisons of 69 populations of the six North American diploid species of Polystichum resulted in new hypotheses of interspecific relationships in this poorly understood species complex. Polystichum lonchitis, circumboreal in distribution, is highly divergent allozymically from the other five species; genetic identities between P. lonchitis and the other species range from 0.187 with P. acrostichoides to 0.378 with P. munitum. Among the strictly North American species, two groups emerged from the analyses, one consisting of P. acrostichoides, P. dudleyi, and P. lemmonii and the other comprising P. imbricans and P. munitum. Genetic identities among these five species range from 0.524 between P. arostichoides and P. imbricans to 0.924 between P. acrostichoides and P. lemmonii, with a mean of 0.666. This mean values is much higher than those obtained in other analyses of genetic divergence among congeneric fern species and suggests relatively recent radiation in North American Polystichum. Because genetically similar species of Polystichum are allopatric or at least allotopic, a model of geographic speciation, perhaps with an edaphic component, is proposed for North American Polystichum.