The analysis of mutants is an indispensable approach towards characterizing gene function. Combining several tools of Drosophila genetics, we designed a new strategy for a mutagenesis screen which is fast, easy-to-apply, and cheap. The combination of a cell-specific Gal4 line with an upstream activating sequence-green fluorescent protein (UAS-GFP) allows the in vivo detection of the cells or tissues of interest without the need for fixation and staining. To further simplify and accelerate the screening procedure, we generated recombinant flies that carry the Gal80 transgene in balancer chromosomes. Gal80 inactivates Gal4; and thus prevents GFP-expression during embryonic and postembryonic development in all individuals carrying the balancer chromosomes. This allows for an easy distinction in vivo between heterozygous and homozygous mutants, the latter being the only ones expressing GFP. Since most of the fly strains and balancer chromosomes can be substituted, this method is suitable for nearly any mutagenesis screen that does not have major restrictions.