Several quantitative traits were studied in Aegilops longissima addition lines and in tetrasomic lines of the common wheat cultivar Chinese Spring. The effects of gene dosage on these traits, which were highly significant and specific for each of the seven homoeologous groups, were more numerous and caused greater differences than the alien genetic effects. It is therefore suggested that in using alien addition lines for evaluating the effect of alien chromosomes on quantitative traits, the homoeologous tetrasomic lines should be used as a control. In comparison to the suggested approach, the use of substitution lines, which are much more difficult to obtain than addition lines, proved, in some cases, less accurate because of the inability of the alien chromosome to fully compensate for the missing homoeologous chromosome.Key words: addition lines, substitution lines, tetrasomes, Triticum aestivum, Aegilops longissima.