The effect of genotype by environment interaction (G x E) was studied in marjoram in an experiment using 34 genotypes and three locations. For yield, no significant G x E was found, while for plant height, flowering time and the essential oil content, the G x E was significant. As to leaf colour, which was determined using the L*a*b* system and a sensorial score, only the coordinate a* was not significant. Both sensorial evaluations for smell and taste revealed some genotypes to be more stable than others. Regarding the essential oil compounds, both of the 'marjoramy' compounds, cis-sabinene hydrate and cis-sabinene hydrate acetate, were extremely unstable in one hybrid combination which, however, turned out to be the most stable comparing to the total amount of these two compounds. The main sesquiterpenes, beta-caryophyllene and bicyclogermacrene, were strongly correlated in their stability, except in one case, where only bicyclogermacrene was highly unstable. The estimation of G x E allows the extrapolation of the degree of instability that is to be expected for a special trait. But, above all, when G x E is given special consideration, it is possible to select for stable genotypes that can be cultivated, e.g. in different regions with the resulting raw material being of higher homogeneity and the environmental influences minimised. Copyright (C) 2003 John Wiley Sons, Ltd.