Escherichia coli beta-galactosidase is a commonly used reporter molecule for analyzing gene expression. Recently, beta-galactosidase fusions have been applied to a variety of eukaryotic systems. The techniques for constructing and introducing beta-galactosidase fusion constructs as well as soluble assays for total enzyme function have been described in detail elsewhere. This article describes histochemical techniques for analyzing organisms that contain a functional beta-galactosidase fusion construct. The object is to determine semiquantitatively which cells are expressing the beta-galactosidase fusion protein, as well as the subcellular localization of the protein. Due to its prevalence in the author's laboratory, Caenorhabditis elegans is used as a canonical organism for the detailed methods described.