The work reports antibacterial and antifungal activity of some 3-(substituted phenyl) isocoumarins (1H-2-benzopyran-1-ones), isocarbostyrils 1(2H)-isoquinolones, the nitrogen analogues of isocoumarins and isocoumarin-1-thiones, the thio derivatives of isocoumarins. The antimicrobial activity was determined against ten different Gram positive and Gram negative bacterial strains and three fungal strains. The bacterial strains were Klebsiella pneumonae (ATCC 6633), Staphylococcus aureus (ATCC 29213), Micrococcus luteus (ATCC 9341), Pseudomonas aeruginosa (ATCC 33347), Escherichia coli (ATCC 25922), Salmonella typhi (ATCC 19430), Lactobacillus bulgaricus, (ATCC 25929), Pasteurella multocida A (ATCC 9150), Staphylococcus epidermidis (ATCC 29232) and Proteus vulgaris (ATCC 49565) and fungal strains were Aspergillus flavus, Aspergillus nigar and Aspergillus pterus. Agar well diffusion method was followed for antibacterial activity and poison plate method was adopted for antifungal assay. Chloramphenicol and fluconazole used as standard drugs for antibacterial and antifungal activity respectively. In general, these compounds exhibited high antibacterial potential than antifungal. Comparative study reveals that the 1-thio derivatives are more active than parent isocoumarins but 1(2H)-isoquinolones, are less active. Most of these compounds showed poor activity but some of these compounds exhibited moderate to good activity against Staphylococcus epidermidis, Klebsiella pneumonae, Escherichia coli and Proteus vulgaris, compared with the standard drug.