ABSTRACT. Green fluorescent protein (GFP) is widely used as a reporter transgene in a variety of organisms. Some of the advantages of using GFP include non-invasive visualization of biological events and/or tissues in live specimens and its benign nature. When GFP is expressed throughout the organism, in neurons and eyes, lifespan and climbing ability of flies are significantly decreased compared to similar crosses with a lacZ reporter. Also, GFP expression can have subtle effects on eye morphology, with neural and ubiquitous expression. Since GAL4/UAS expression of GFP can influence aging and climbing ability in the Drosophila system of directed gene expression, we found that the latter of these advantages, namely its harmless, non-toxic nature, can be conditional, depending upon the mode of expression and the biological endpoint. We suggest that caution should be used when using GFP to visualize cellular events, especially in long-term assays.