Endophytic fungi residing in Panax quinquefolium (American ginseng) have not been well characterized. We collected American ginseng of one-, two-, three- and four-year-old plants cultivated in a forest reserve and identified the endophytic fungal isolates using traditional methods. The colonization frequency and the dominant endophytic fungal species were investigated. Endophytic fungal diversity indices and similarity coefficient were also assessed and all age groups of American ginseng studied were colonized by endophytic fungal assemblages; 134 fungal isolates were assigned to 27 taxa. The infection frequencies varied with the host age and tissue. The dominant endophytic fungi were recorded for each tissue and age of host. The roots of two- and four-year-old American ginseng exhibited the highest and the lowest Shannon-Wiener index respectively. Four-year-old American ginseng had a low similarity coefficient when compared with each of the other three ages classes. The possible role of endophytic fungi in relation to American ginseng cultivation is discussed.