The study of DC-, low and radiofrequency AC-conductivity in combination with Hall and Seebeck coefficients and magnetic measurements have been carried out at temperatures 1.6-300 K, in magnetic field up to 12T in the so-called Kondo-insulator compound-narrow-gap semiconductor FeSi. The experimental data obtained on high-quality single crystals of iron monosilicide allow us to conclude in favor of Mott-Hubbard scenario of metal-insulator transition with the on-site Coulomb interaction U in vicinity of the critical value U less than or equal to U-c approximate to 3D (D- is the band half-width). The estimation of microscopic parameters provides additional arguments in favor of the spin polarons formation and dramatic renormalization of the intra-gap electronic density of states in FeSi at temperatures below 100K. In the temperature interval Tless than or equal toT(c)approximate to15 K an onset of the coherent spin fluctuations occurs in the cubic lattice of FeSi resulting in the formation of ferromagnetic microregions (about 10 Angstrom in size). When the temperature decreases below T(m)approximate to7 K an interaction between these ferromagnetic particles causes a mictomagnetic transition in this low carrier density (10(17)-10(18) cm(-3)) d-electron material. (C) 2002 Elsevier Science B.V. All rights reserved.