Extensive random sampling of populations of Thuja occidentalis growing on vertical cliffs of the Niagara Escarpment in southern Ontario, Canada, was conducted to determine the extent of an old-growth forest that had recently been described. Nine sites distributed along the length of the escarpment were intensively sampled and from these, 1254 increment cores or cross sections were obtained from 872 trees in all age categories. The results show that all cliffs support a broadly similar old-growth forest of stunted trees, but that statistically significant differences were found in the numbers of trees among sites. No large differences among sites were found in heights or diameters of trees. Maximum ages of 532 (sampled) and 814 years (estimated) were found in the random census, although in subsequent selective sampling, intact stems up to 1032 years were found. The incidence of fire and cutting by humans was also measured, but little evidence of such disturbances was found. It is concluded that exposed cliff faces of the Niagara Escarpment support one of the oldest, most extensive, and most intact old-growth forest ecosystems yet described for eastern North America. The opportunities for the study of basic forest ecology and especially for dendrochronology are considerable. Key words: Thuja occidentalis, old growth, Niagara Escarpment, cliff, age structures.