Ploidy level was determined for six species and 88 cultivars of the Rhododendron subgenus Tsutsusi. High-resolution flow cytometry of nuclear DNA was performed on macerated plant tissue. All plants analyzed were diploid (2n = 26) with the exception of `Euratom', `Euratom Orange', and `Red Wing', which were triploid (3n = 39), and `Casablanca Tetra', which was found to be a cytochimera: mixoploid (2n + 4n) in the LI and LII, but tetraploid in the LIII. The described method has proven to be useful in screening a large population of rhododendrons. Analysis of different organs and plant tissues was easily accomplished through flow cytometry, and has proven useful in determining the ploidy of different histogenic layers.