Efficient negative selection systems are increasingly needed for numerous applications in plant biology. In recent years, various counter-selectable genes have been tested in six dicotyledonous species, whereas there are no data available for the use of negative selection markers in monocotyledonous species. In this study, we compared the applicability and reliability of two different conditional negative selection systems in transgenic barley. The bacterial codA gene encoding cytosine deaminase, which converts the non-toxic 5-fluorocytosine (5-FC) into the toxic 5-fluorouracil (5-FU), was used for in vitro selection of germinating seedlings. Development of codA-expressing seedlings was strongly inhibited by germinating the seeds in the presence of 5FC. For selecting plants in the greenhouse, a bacterial cytochrome P450 mono-oxygenase gene, the product of which catalyses the dealkylation of a sulfonylurea compound, R7402, into its cytotoxic metabolite, was used, T1 plants expressing the selectable marker gene showed striking morphological differences from the nontransgenic plants. In experiments with both negative selectable markers, the presence or absence of the transgene, as predicted from the physiological appearance of the plants under selection, was confirmed by PCR analysis. We demonstrate that both marker genes provide tight negative selection; however, the use of the P450 gene is more amenable to large-scale screening under greenhouse or field conditions.