Imperfections relating to iron contamination in undoped CuInSe2 single crystals have been studied using Electron Spin Resonance at liquid He temperature. A fine structure of the S=5/2 state attributed to Fe3+ has been observed for crystals of Cu/In ∼ 1. On the other hand, some of indium-rich (Cu/Ing eff∼10. Fe2+ is one of the possible candidates for this line. The two different ionization states of iron in CuInSe2 suggest that iron atoms substitute copper sites in Cu-deficient system rather than indium sites which are preferably occupied by Fe3+ in nearly stoichiometric system.