A new strategy, enhancer trapping, provide a novel way to detect in situ a gene activity involved in the neurogenesis of Drosophila. E. coli lacZ gene fused with a weak promotor of Drosophila are randomly integrated into the genome. lacZ is expressed in a spatio-temporal-specific mammer under the control of the enhancer located nearby an inserted sequence. The applications of this new methd to study the postembryonic development in Drosophila are discussed.