The strong drive to exploit wind energy has recently led to consideration of new types of location for wind turbines, including mountain regions. A major concern for wind farm installation in these sites is the long-term reliability of the support structures. In a flexible steel tower, a combination of vortex shedding and gusting caused fatigue cracks at the base joint. To identify the conditions critical to the development of this phenomenon, a two-blade down-wind turbine was thoroughly investigated in an in-field experimental campaign. This turbine features a 13m diameter rotor of rated power 11kW, mounted on an 18m tubular steel tower. In operation, the blades rotate at a fixed rate of 2 Hz. The tower, instrumented with 15 accelerometers, was first dynamically characterized in the absence of significant wind. Next, its spectral response to wind excitation was identified both in operation and with the rotor at rest. The outcome of the experiment suggests that the vulnerability to fatigue of this model of turbine is very sensitive to its modal behaviour, this in turn depending on the mechanical admittance of the foundations.