Two isolates of peanut stripe virus (PStV), stripe and blotch, were compared ultrastructurally in peanut (Arachis hypogaea L. 'Florigiant') at several stages of leaf expansion. Ultrathin sections of young leaves infected with either isolate of PStV revealed pinwheel inclusions attached to the cell wall near plasmodesmata. The cytoplasm of infected cells were highly vesiculated. Virus particles amassed in crystalline arrays were observed in blotch infected cells. Virus particles were observed along the arms of pinwheel inclusions. Scroll inclusions appeared in PStV infected cells at a later stage of leaf expansion. In more mature leaves, pinwheel and scroll inclusions occurred in the cytoplasm in association with mitochondria. Virus particles were observed free in the cytoplasm as well as concentrated in linear arrays along the inner surface of the tonoplast. Membrane and organelle degradation was evident in cells infected with either isolate of the virus. Numerous cytoplasmic inclusions and virus particles were observed in cells from light green areas of the leaf. Cells from dark green areas did not contain cytoplasmic inclusions and contained few if any virus particles. Particle measurements show stripe and blotch isolates to have a mean length of 753 nm and 747 nm for leaf dip preparations and 746 nm and 745 nm for partially purified preparations, respectively. Both isolates had a modal length of 750 nm, regardless of the extraction procedure. The relative virus titer of each isolate was determined in peanut leaves at five stages of leaf expansion and in dark green and light green areas of infected leaves. Virus titer increased significantly from the closed to the fully expanded stage, at which time the virus titer peaked and then decreased slightly. Virus titer was consistently higher in leaves infected with the blotch isolate at all expansion stages. Virus titer was also higher in cells from light green areas of the leaf than from dark green areas of the leaf, regardless of isolate. / M.S.