Department of Chemistry, University of Rajasthan, Jaipur-302 004, India E-mail : niilima@sancharnet.in; rajbns@sify.com Manuscript received / September 2003 Ab initio quantum chemical calculations of the indolizine, aza-, diaza- and triazaindolizines and their various possible mono phospha-analogues obtained by replacement of one =CH— in the five membered ring by sp2-phosphorus (in all 20 systems : A-T) have been carried out at the B3LYP/6-311G** hybrid density functional level of theory. Thus different parameters of the molecular geometries and electronic structures, namely bond lengths, bond angles, total atomic charges, total energies, dipole moments and energies of the frontier molecular orbitals have been determined for these systems. σ2,λ.3-Phosphorus is found to be well integrated with the aromatic system which is further supported by NBO calculations of A-C. Although introduction of theσ2,λ.3-phosphorus in the five membered ring causes perceptible changes in the geometrical and electronic parameters, the aromatic character is comparable with that of the indolizine.