Allopolyploidy is common in the homosporous fern Polystichum and may account in large part for the taxonomic complexity of the genus. Six polyploid species of Polystichum occur in North America. We addressed the origins of two tetraploids (2n = 164), P. californicum and P. scopulinum, using allozymic data for 12 loci and restriction fragment analyses of chloroplast DNA (cpDNA). Two hypotheses have been advanced for the origin of P. californicum: 1) the diploid progenitors are P. dudleyi and P. munitum; and 2) P. californicum is polyphyletic, with populations from California and southern Oregon derived from P. dudleyi and P. munitum and those populations from northern Oregon and Washington derived from P. dudleyi and P. imbricans. Allozymic data confirm the allotetraploid nature of P. californicum; fixed heterozygosity was observed at six loci, although only two or three loci were fixed in most populations. Polystichum californicum contains the genome of P. dudleyi, and allozymic data suggest that P. imbricans contributed the second diploid genome. cpDNA data confirm the role of P. imbricans in the origin of P. californicum; all populations of P. californicum examined, from throughout its range, possessed the chloroplast genome of P. imbricans. Polystichum scopulinum, distributed at high elevations throughout much of western North America, was also confirmed to be an allotetraploid, displaying fixed heterozygosity at six loci, although no more than five loci were fixed in any one population. Allozymic data indicate that P. lemmonii was one of the diploid parents of P. scopulinum. The chloroplast genome of all populations of P. scopulinum examined was also contributed by P. lemmonii. Allozymic data also implicate P. imbricans as the other diploid parent, rather than P. munitum as some have proposed.