Old-growth ponderosa pine (Pinus ponderosa) forests are uncommon in the Southwest, and only one old-growth forest (the Gus Pearson Natural Area [GPNA]) has been researched in the ponderosa pine belt surrounding the city of Flagstaff in northern Arizona. The purpose of this study was to measure soil characteristics, current and pre-Euro-American settlement (1885) tree structure, and understory plant composition in a 6-ha remnant old-growth forest on volcanic, red cinder soils. Soil bulk density was extremely low (0.21 Mg/m³) in this forest because of high volumetric contents of cinders >2 mm diameter. As a result, volumetric soil moisture, organic C, and total N contents were low, with June gravimetric moisture (0–15 cm) averaging <1%. Despite these seemingly inhospitable soils, the reconstructed ponderosa pine presettlement density of 183/ha is among the highest reported for northern Arizona. Current density of live presettlement-origin trees also is high (104/ha), including 36 trees/ha that established before 1700. On a 1-ha plot, the live tree age structure reconstructed for 1885 suggested that all 29 decades between 1600 and 1890 had at least one tree establish. These temporal establishment patterns are more constant than those reported at the GPNA, but do support GPNA findings of uneven-agedness within tree groups. Plant communities were dominated by mountain muhly (Muhlenbergia montana) and other species of xeric affinity. Several ecological properties at this site differed sharply from the GPNA, which occupies moist basalt soils, and the site is a member of a red cinders/Bahia ecosystem type that is among the rarest in this region.