Depending on genotype, sugar beet can differ considerably in yield and quality characteristics. These are additionally modified by environmental conditions with drought stress recently gaining in importance, restricting growth and altering the chemical composition of the beet. The occurrence and development of these genotypic differences during the vegetation period and their possible interaction with environmental conditions were investigated. In 2002 and 2003, four sugar beet genotypes differing in yield and quality and putative different with regard to drought tolerance were tested in field trials, partly under irrigated conditions, in a total of 10 environments with consecutive harvests starting in early summer. In 2 years of stress and non-stress conditions they exhibited significant differences for taproot and leaf dry matter and the concentration of sucrose, K, Na and a-amino nitrogen in the taproot. These differences existed already in mid-June and virtually did not change any more from this time on. Accordingly, interactions between genotype and harvest date did not occur. For sugar beet, genotype by environment interactions generally do not exist. Water supply, as an important single determinant of the effect of the environment, was studied separately analysing data from selected locations. Under drought conditions, withholding irrigation reduced leaf and taproot growth and root-to-leaf ratio, decreased the percentage of sucrose in dry matter and resulted in an accumulation of a-amino N. Interactions between genotype and water supply did not occur for any of the parameters under study. A genotype-specific high a-amino N content, which might be of advantage for osmoregulation, did not improve the adaptation to drought. Differences in leaf maintenance or taproot-to-leaf ratio during drought also did not affect yield response. Due to the lack of interaction between genotype and harvest date as well as between genotype and irrigation it is concluded that harvest date or climatic factors of the growing region do not have to be taken into consideration when choosing a variety.