In a continuing chromosome survey of Pycnanthemum, chromosome counts are given for 21 accessions representing 12 species. Among these is a first record (2n = 78) for P. floridanum and a corrected diploid count (2n = 38) for a collection of P. loomisii earlier reported to be tetraploid. The first hexaploid count (2n = 120) known for P. torrei supplements prior reports of tetraploidy in this taxon. Data on chromosome numbers, meiotic behavior, and percent stainable pollen are presented for 16 interspecific crosses that produced flowering F1 hybrids. Thirteen species were used in the hybridization program, including eight diploids representing six distinct species groups. In three species combinations, only non-flowering F1 hybrids were formed. Although the species of Pycnanthemum are capable of crossing even in morphologically widely divergent combinations, the hybrids are generally characterized by meiotic abnormalities and low pollen stainability.