Sato et at. [1987> recently drew attention to a new form of Pc 1 pulsations, which they refer to as ''fingerprint structure' Pc 1 pulsations because of their distinctive appearance in frequency-time (f-t) spectrograms. The new pulsations were observed on moderately high latitude geomagnetic field lines (L≈6) and the authors suggested that the considerable number of spectral lines and their constant frequency separation might provide evidence for non-linear effects in the Pc 1 pulsation generation region. It is suggested here, on the contrary, that the fine structure is not a non-linear effect but is simply a manifestation of field line resonances of the kind suggested by Fraser-Smith [1967>. ¿ American Geophysical Union 1987