  We calculate exactly the functional determinant for fermions in fundamental
representation of SU(2) in the background of periodic instanton with
non-trivial value of the Polyakov line at spatial infinity. The determinant
depends on the value of the holonomy v, the temperature, and the parameter
r_12, which at large values can be treated as separation between the
Bogomolny--Prasad--Sommerfeld monopoles (or dyons) which constitute the
periodic instanton. We find a compact expression for small and large r_12 and
compute the determinant numerically for arbitrary r_12 and v.
