This chapter discusses the struction of the monopartite elongated viruses containing single-stranded RNA genome of positive polarity and placed in five groups: tobamoviruses, potexviruses, carlaviruses, potyviruses, and closteroviruses. Tobacco mosaic virus (TMV) was the first virus to be crystallized and also the first virus to be studied using an electron microscope. The TMV particles consist of coaxially intertwined protein and RNA to produce helical, grooved, cylindrical, rigid rods of remarkably uniform dimensions of 300 nm length and 18 nm diameter. The capsid of each TMV particle is composed of about 2130 identical protein subunits. Intact TMV particles are readily dissociated into their two constituent parts by acetic acid treatment or in cold dilute alkaline solution. Hydrophobic interactions between protein subunits are the predominant bonds stabilizing TMV virions. Potyviruses are elongated flexuous viruses with length in the range of about 720 to 850 nm, but the length of most potyviruses is between 720 and 770 nm.