The frequency and intensity of harmful cyanobacterial blooms (HCBs) are increasing all over the world, their prevention and control have become a great challenge. In this paper, a series of 1,3,4-thiadiazole thioacetamides (T series) were designed and synthesized as potential algaecides. Among them, the compound T3 showed its best algacidal activity against Synechocystis sp. PCC 6803 (PCC 6803, EC50 = 1.51 mu M) and Microcystis aeruginosa FACHB 905 (FACHB905, EC50 = 4.88 mu M), which was more effective than the lead compound L1 (PCC6803, EC50 = 7.7 mu M; FACHB905, EC50 = 8.8 mu M) and the comm...