Positive staining. This is the most frequent artifact in virus electron microscopy. Uranyl salts cause both negative and positive staining, while phosphotungstates and molybdates cause negative staining only ( Table 1) . The principal incriminated stain is uranyl acetate (UA). In positive staining (Figs. S9 and S13), virus particles themselves are stained and then may appear black on a white background. This is due to the strong affinity of uranyl ions to dsDNA 13 and seen in any viruses with compact masses of DNA, e.g., phage heads and adeno-or herpes-viruses. It never occurs in filamentous or ssRNA phages. Positively stained phage heads are invariably shrunken by about 10-15% 5, 16 and show neither capsomers nor transverse edges. Phage tails are not stained and appear as shadows. Consequently, these viruses can rarely be identified by EM. Their dimensions are perfectly useless and should not be published. If positive staining is generally an undesirable artifact, it has however two important applications. It allows (A) the visualization of phage heads in sectioned bacteria and (B) facilitates the quantification of aquatic viruses. 4 The reasons for positive staining are unknown. Both negative and positive staining may occur in adjacent areas of the same EM grid (Figs. S10, S12 and S13). The addition of protein to UA solutions seems to alleviate the incidence of positive staining.