In order to understand the symmetry, structure and defects of Mn(II) impurity incorporated in diaquabis[malonato(1-)-κ 2 O,O′] zinc(II), single crystal EPR studies have been carried out at X-band frequencies at room temperature. Angular variation in the three orthogonal planes shows the presence of two defects, having a relatively large and small zero-field splitting parameter (D), with orthorhombic symmetry. The spin-Hamiltonian parameters, estimated from the three mutually orthogonal crystal rotations, are:g xx =1.959, g yy =1.998, g zz =2.011; A xx =−8.73, A yy =−8.55, A zz =−9.10mT; D xx =5.58, D yy =1.33, D zz =−6.91mT.g xx =2.015, g yy =1.996, g zz =2.004; A xx =−8.18, A yy =−8.00, A zz =−8.58mT; D xx =33.48, D yy =6.92, D zz =−40.40mT.The observed large zero-field tensor for Defect II is due to the steric effects caused by the two malonate rings. The location of the two interstitial defects has been determined from the X-ray data of the host lattice. The powder EPR spectrum also confirms the chemical inequivalence of the two defects. The optical absorption spectrum shows the characteristic of Mn(II) ions in distorted octahedral symmetry.