The Hepaticae of western North America, in contrast to eastern North America and many other parts of the world, are poorly known. This present flora for southwestern British Columbia, Canada, and part of northwestern Washington State, U.S.A., is intended to partially alleviate this problem. The geographical area covered in this study offers great diversity in topography, climate, and vegetation. The region includes the wet Pacific coast of western Vancouver Island, mesic forests and floodplains of low elevations, the dry area around southern Georgia Strait, subalpine forests and meadows of the Coast Range and North Cascade Mountains, alpine soil and rock substrates at highest elevations below summer snowline or glaciers, and dry habitats in the rainshadow northeast of the mountains. The material presented is based on direct study of approximately 4000 personal and 1800 herbarium collections of hepatics. A total of 166 species and two additional varieties, belonging to 64 genera in 37 families, are treated in keys and concise descriptions, accompanied by ecological and phytogeographical information. Additional discussion on taxonomic and systematic difficulties is given where pertinent. Five hepatics new to science were discovered during this study, with Schofieldia monticola Godfr. having been recently described. Hepatics collected which had not been reported previously from continental North America include Eremonotus myriocarpus (Carring.) Lindb. et Pears, in Pears., Jungermannia hattoriana (Amak.) Amak., Marsupella condensata (Ångstr. in Hartm.) Schiffn., and Nardia japonica Steph. Hepatics belonging to, or similar to, Lophozia elongata Steph. and Marsupella adusta (Nees emend. Limpr.) Spruce were collected. Hepatics new to British Columbia include Lophozia ventricosa var. silvicola (Buch) Jones, Riccia frostii Aust., Scapania gymnostomophila Kaal., and S. paludlcola Loeske et K. Müll, in K. Müll. For the first time, fasciculate gemmae were discovered in Ghandonanthus Mitt., and inflated lobules were found in Porella navicularis (Lehm. et Lindenb.) Lindb. Based on specimens exajnined from the study area, Bazzania ambigua (Lindenb.) Trev., Odontoschisma gibbsiae Evans, and Plagiochlla satoi Hatt. were treated as synonyms of other species in this flora. Systematic problems requiring detailed future study were encountered in particular in Bazzania S. Gray and Galypogeia Raddi emend. Nees. Regional distribution-maps depicting all known points of occurrence and general elevation are presented for all hepatics discussed in this flora. Four general categories of distribution were demonstrated by a comparison of the spatial patterns: l) distributions in moist, humid climates influenced to varying degrees by the Pacific Ocean (18% of the total flora), 2) high elevations in the Coast and Cascade Mountains (24%), 3) dry climates in rainshadow areas (16%), and 4) various types of widespread distributions (33%). The remaining 9% of the species are rare or infrequent and cannot be assigned to any particular category. The hepatic taxa were assigned to 14 phytogeographical elements based on total worldwide distributions. Approximately 50% of the hepatics have essentially circumboreal distributions. This figure includes taxa missing from eastern Asia. Ten percent of the hepatics are endemic to western North America. Of the hepatics demonstrating disjunct, or discontinuous distributions, 16% have European affinities, 9% eastern Asian affinities, and 11%, affinities with both regions. Approximately 11% of the hepatics treated in this flora have bipolar disjunctions. In an attempt to reconstruct some events in the development of the modem hepatic flora of southwestern British Columbia, possible Pleistocene refugia and their effect on hepatic populations are discussed. This flora will provide a manual for the Hepaticae and Anthocerotae of southwestern British Columbia, and will serve as a preliminary guide to these plants in the North American Pacific Northwest. / Science, Faculty of / Botany, Department of / Graduate