The spectral type of HD 166596 (HR 6804) is given as B2.5III in The Bright Star Catalogue (Hoffleit and Jaschek 1982) and as B2HIII by Houk (1978). Its rotational velocity, Vsin i, is 255 km s-1 (Uesugi and Fukuda 1982). Neubauer (1930) noted that its radial velocity is probably variable. Bidelman and MacConnell (1973) designated this star as a silicon star, and thereafter, Renson and Manfroid (1980) found its photometric period, P = 1.67 ± 0.01 days (or 0.83 days) with the amplitudes of 0ṃ04 ∼ 0ṃ05 from their four-color photometry. To the best of our knowledge, there is no report on the presence of any emission line in the foregoing literature.