Several trees were produced by tissue culture of shoot tips from each of 5 putative Euramericana hybrid poplar clones and 1 Lombardy poplar clone. Cuttings from each tissue-cultured tree were established to estimate variation among tissue-cultured derivatives of each parental clone. A wide range of variation was found for height, number of branches, and leaf traits after 1 growing season. Heat treatment of shoots, to aid in freeing clones from viruses before culturing, did not influence clonal vigor in a consistent way. Chromosome counts of roots from many ramets were highly variable while the few counted leaf cells were diploid. Instability of chromosomal numbers in roots is one possible cause of extremely reduced vigor of some tissue-cultured trees. Clonal testing of trees derived by tissue culture is necessary unless the cause of within-clone variation can be identified and eliminated. Forest Sci. 23:122-131.