The class I myosins are single-headed, actin-binding, mechanochemical "motor" proteins with heavy chains in the molecular mass range of 110-130 kDa; they do not form filaments. Each myosin I heavy chain is associated with one to six light chains that bind to specific motifs known as IQ domains. In vertebrate myosin I isoforms, the light chain is calmodulin, which is thought to regulate motor activity. Proteins similar to calmodulin are associated with myosin I isoforms from lower eukaryotes. Some myosin I isoforms from lower eukaryotes are regulated by phosphorylation; however, the phosphorylation site is not present in vertebrate myosin I isoforms. Based on sequence analyses of the amino terminal "head" domains, myosin I can be subdivided into several subclasses. Analyses of the biochemical properties of the isolated molecules and localization studies support the proposal of roles for these molecules in intracellular trafficking and changes in membrane structure. Our present understanding of the properties of these molecules and their proposed roles is reviewed here.