ULF observations at two mid-latitude sites during the large geomagnetic storm of July 15-16, 2000 were used to trace variations of resonance frequencies of the field line resonators. A brief description of the geomagnetic disturbance as it was observed on the ground, at the geostationary orbit, and before the Earth’s bow shock is given. Then a detailed study of ULF dynamic spectra from Borok and Mondy is performed for 8 succesive 6-hour intervals of July 15 and 16. In conclusion some tasks for the future work are listed. ULF observations at two mid-latitude sites during the large geomagnetic storm of July 15-16, 2000 were used to trace variations of resonance frequencies of the field line resonators. A brief description of the geomagnetic disturbance as it was observed on the ground, at the geostationary orbit, and before the Earth’s bow shock is given. Then a detailed study of ULF dynamic spectra from Borok and Mondy is performed for 8 succesive 6-hour intervals of July 15 and 16. In conclusion some tasks for the future work are listed .