In order to study gene expression in the human parasite Entamoeba histolytica, a cDNA library of E. histolytica strain 200:NIH was constructed using the phage vector lambda gt10. Three cDNA clones (A, B and C) were selected for further analysis. Each of the three clones hybridized to a distinct mRNA. Two of these mRNAs were translated in vitro after hybrid selection, and yielded distinct translation products. One of these mRNAs, selected by hybridization to clone A, encodes the most abundantly expressed protein in E. histolytica. DNA sequence analysis of this cDNA clone identified the DNA as that encoding actin. The deduced amino acid sequence of E. histolytica actin resembles both cytoplasmic and muscle actins and has an unusual N-terminal glycine residue. We have shown that a family of actin genes is present in E. histolytica. Six different E. histolytica actin clones were obtained from a lambda gt10 genomic library using subcloned cDNA probes. Southern analysis of three different E. histolytica strains (200:NIH, Rhaman, and HM-1:IMSS) revealed at least four different actin genes. Strain HM-1:IMSS, however, differs by the presence of an additional actin gene.