The structures of anthracene adlayers on Ag(111), Ag(100), and Ag(110) electrode surfaces in dilute perchloric acid have been investigated in detail by using in situ scanning tunneling microscopy (STM). Anthracene was found to form highly ordered molecular layers with a flat-lying orientation on the Ag single-crystal electrode surfaces in the double-layer region. High-resolution STM images allowed us to reveal packing arrangements of anthracene. Highly ordered anthracene adlayers with (2 root3 x 2 root3)R30 degrees (theta = 0.08), c(4 x 6) (theta = 0.08), and c(4 x 4) (theta = 0.125) symmetries were observed on Ag(111), Ag(100), and Ag(110) surfaces, respectively.