Escherichia coli lacZ, which encodes beta-galactosidase, has become a widely used reporter gene to study the developmental regulation of gene expression in a variety of organisms. To detect the presence of the beta-galactosidase, the sample must be fixed and appropriately stained. This sort of analysis yields rather crude estimates of the spatial-temporal changes in gene expression patterns. In addition, one cannot recover interesting specimens for propagation. A novel fluorogenic beta-galactosidase substrate for use in live Drosophila melanogaster embryos has been designed and synthesized. This compound provides a means to determine gene dosage in live embryos so that one can unambiguously determine the genotype of a living embryo. This will be useful for detailed analysis of cellular and morphogenetic behavior changes in live embryos that are homozygous for embryonic lethal mutations. In the course of testing this compound, a new beta-galactoside hydrolytic activity, different from the previously identified beta-galactosidase, has been discovered to reside in macrophages and the intervitelline space.