Using minimum-dose system and optical diffraction, effects of electron irradiation on negatively stained images of trypsin-straightened nucleocapsids of Sendai virus were semiquantitatively compared for uranyl acetate (UA) and phosphotungstic acid (PTA). The results confirmed the superiority of UA in display of fine structures and showed that both UA- and PTA-stained images tended to turn from a one-sided to a two-sided image during irradiation, the general contrast of the picture increased in the UA-stained images but not in the PTA-stained ones, and furthermore the electron doses for the richest information were 18 000 to 30 000 e −/nm 2 for UA, but 1000 e −/nm 2 for PTA, under the condition used. The optical diffraction patterns of the UA-stained nucleocapsids, its analysis by the superposition method, and rotational harmonics of end-on views of nucleocapsids, together indicated that the most probable arrangement of subunits was 13 per turn of a helix with 5-nm periodicity. This helix also had an arrangement of subunits parallel to the axis. The occurrence of 2.5 nm periodicity was probably produced by an arrangement of a UA-penetrable concave substructure of the subunit.