The central role of gene expression in regulating development has largely been studied by in situ hybridization and antibody staining techniques in fixed material. However, rapid temporal and spatial changes in gene expression are often difficult to correlate with complex morphogenetic movements. A green fluorescent protein (GFP) from the jellyfish, Aequorea victoria, can be used as a real-time reporter for gene expression and could aid analysis of dynamic events during embryogenesis. Here, we describe a transgenic Drosophila line ubiquitously expressing a nuclear GFP fusion protein that highlights morphogenesis, cell movement, and mitosis in living embryos. The fusion protein is highly fluorescent when maternally supplied, but there is a long delay between its zygotic expression and the appearance of fluorescence. GFP is thus an excellent marker for the expression of stable gene products, but a poor reporter for dynamic zygotic gene expression in early Drosophila embryos.