SUMMARY: The genus Mallomonopsis (Chrysomonadales) is discussed and five taxa are described from observations with the electron microscope: M. elliptica, M. elliptica var. salina, M. elliptica var. oviformis (formerly Mallomonas oviformis), M. peroneides sp.nov. and M. ouradion (formerly Mallomonas ouradion). The two flagella, which distinguish Mallomonopsis from Mallomonas, comprise a stout one with mastigonemes and a slender one which is smooth and has a photoceptor at its base. The silica scales are compared in detail and are shown to differ in construction as well as in appearance in the different species and are considered to provide the only satisfactory basis for specific separation.