New series of [1,3,4]thiadiazolo[3,2-a]pyrimidines, benzo[h][1,3,4]thiadiazolo[2,3-b]quinazolines, benzothiadiazolotriazocine, and imidazo[2,1-b][1,3,4]thiadiazoles have been synthesized and characterized by analytical and spectrometrical methods (IR, MS, (1)H and (13)C NMR). Twenty of the synthesized compounds were screened for antibacterial activity against Escherichia coli, Staphylococcus aureus and Bacillus cereus. They were found to be either moderately active, slightly active or inactive against the tested microorganisms. The antifungal activity of these compounds were also tested against Candida albicans, Aspergillus fumigatus 293 and Aspergillus flavus 3375. Compounds 3g, h showed potent antifungal activity against C. albicans. In addition, the same compounds were tested for antiquorum-sensing activity against Chromobacterium violacium ATCC 12472, where compounds 3b,c, 3f-h, 6b-d, 9, 10 and 12 demonstrated acceptable activity. Compounds 3d, 9 and 10 were screened for antitumor activity at National Cancer Institute, USA. The in vitro cytotoxic activity of eighteen of the synthesized compounds was studied by brine shrimp lethality bioassay, and results indicated that compounds 6c, 13, 3h, 6d and 3d have the highest cytotoxic activity.