New complexes of N-isonicotinamido-2-hydroxy naphthalaldimine with Co(II), Zn(II), and Cu(II), having formula of the type [M(L)2].nH2O (M= Zn+2, n=2, Co+2, n=1, Cu+2, n=3), and with Ni(II), having formula of the type [M(L)H2O](ac).nH2O (M= Ni+2, n=0.5 ac = CH3COO–) have been synthesized. The nature of bonding and geometry of the transition metal complexes as well as the Schiff base ligand have been deduced from NMR, IR,UV-VIS, AAS, XRF, EI mass spectrometry, elemental analysis and thermo gravimetric studies. Co(II), Zn(II), and Cu(II) metal ions formed 1:2 (M:L) complexes; whereas, Ni(II) gave a 1:1 (M:L) complex. Based on elemental and spectral studies, octahedral geometries were assigned to Zn(II), Co(II) and Cu(II) complexes and a tetrahedral geometry was assigned to Ni(II) complex. The ligand acts as a monoanionic tridentate and coordinates through the carbonyl oxygen, azomethine nitrogen and phenolic oxygen. The biological activity of the ligand and of the corresponding complexes was tested on Gram positive bacteria (Escherichia coli) and Gram negative bacteria (Staphylococcus aurous) and two fungi (Aspergillusflavus and Candida albicans). The biological activity data show that some metal complexes are more potent than of the free ligand