Continuing with the pioneering work by M. Graham Netting, the abundance of woodland box turtle (Terrapene carolina carolina) and wood turtle (Glyptemys insculpta) were opportunistically monitored for six decades (1959–2019) at the Powdermill Nature Reserve (PNR) in southwestern Pennsylvania. We found that both of the species grew slowly with delayed maturity and potentially long lives. Four wood turtles were recaptured 27–45 yrs after first marking, and five box turtles were 28–45 yrs post-marking. Both sexes of each species had high adult survivorship, with females exhibiting higher recapture probabilities. Although we found relatively few box turtles at PNR, their numbers were generally steady over six decades. Conversely, the wood turtle was frequently encountered in the 1960s but then had an apparent decline with no recovery. For example, of the 161 marked wood turtles, most (57.8%) were encountered during the first decade of study and only 15 were marked since 2004. We found no relationship between wood turtle numbers and major weather events such as severe cold periods or flooding. Thus, several factors may have coalesced to reduce their numbers at PNR: increased vehicle use of a road that bisects the property, habitat succession, stream pollution from prior coal mining, and collecting for the pet trade. As protective measures for these terrestrial turtle species, we suggest the construction of fences and under-road culverts for safe passage across roads, and the restoration of patches of open habitats across the property for turtle nesting.