A plasmid vector that serves as a dominant marker for isolating transformed animals in Caenorhabditis elegans has been constructed as a translational fusion of the C. elegans act-4 gene (encoding actin) and the Escherichia coli lacZ gene. This gene fusion can be used as a marker in transformation rescue experiments in any fertile strain of C. elegans. Progeny of animals injected with the act-4::lacZ fusion vector are stained histochemically with XGal, and transformants turn blue. The internal eggs of stained animals remain viable, allowing recovery of the transformed strain. When the act-4::lacZ vector is co-injected with an unselected plasmid with which it shares some sequence homology, most transformants that are recovered by screening for expression of the act-4::lacZ fusion contain both plasmids. Production of active beta Gal in animals transformed with the act-4::lacZ gene fusions appears to be limited to certain tissues. A chimeric gene that contains the 5' and 3' regions of act-4 is expressed strongly in the body-wall muscles, vulval muscles, and spermathecae. Addition of the internal portion of act-4, including the protein-coding region and introns, to this chimeric gene leads to additional lacZ expression in the pharynx.