The age, size structure and the spatial pattern of major tree species were investigated in a temperate old-growth (ca. > 300 years old) Chamaecyparis obtusa (Sieb. et Zucc.) Endlicher forest in the Akasawa Forest Reserve of Central Japan. All stems greater than or equal to 5cm diameter at breast height (dbh) were mapped on a 4-ha plot and analyses were made of age, size structure and the spatial distribution of stems for three height classes of six species. The dominant species was C. obtusa, followed by Thujopsis dolabrata Sieb. et Zucc., C. pisifera (Sieb. et Zucc.) Endlicher and deciduous broad-leaved trees (DBL) species such as Quercus mongolica Fischer ex Turcz., Magnolia obovata Thunb. and Betula grossa Sieb. et Zucc. The dbh and age distribution of C. obtusa and C pisifera was bimodal and rather continuous; two age modes were about 50 and 300 years ago. That of T dolabrata and M. obovata was reverse J-shaped or L-shaped, and of Q. mongolica and B. grossa was bimodal and discontinuous. Canopy stems of C obtusa were distributed randomly throughout the plot, although those of other species and understory stems of every species were distributed patchily, and were often related to forest floor conditions or canopy gaps. Intraspecific spatial, positive associations between canopy and understory stems could be found for only T dolabrata at smaller scales. Notable positive associations between canopy and understory stems were not found for any combination of species at any quadrat size. These results suggest that, in the canopy layer of this forest, C obtusa will become less important and T dolabrata, which is more shade tolerant, will become more important. The DBL species which are less shade tolerant than the conifers, especially M. obovata, will also become more important, depending on forest floor conditions and/or gap formation. (C) 2001 Elsevier Science B.V. All rights reserved.