The simple eukaryote Dictyostelium discoideum contains at least 12 unconventional myosin genes. Here we report the characterization of one of these, myoJ, a gene initially identified through a physical mapping screen. The myoJ gene encodes a high molecular weight myosin, and analysis of the available deduced amino acid sequence reveals that it possesses six IQ motifs and sequences typical of alpha helical coiled coils in the tail region. Therefore, myoJ is predicted to exist as a dimer with up to 12 associated light chains (six per heavy chain). The 7.8 kb myoJ mRNA is expressed all throughout the life cycle of D. discoideum. The myoJ gene has been disrupted and a phenotypic analysis of the mutant cells initiated. Finally, phylogenetic analysis of the head region reveals that myoJ is most similar to two plant myosin genes, Arabidopsis MYA1 and MYA2, that have been alternatively suggested to be either members of the myosin V class or founding members of the myosin XI class.