Thirty new pyrazoline derivatives were synthesized by cyclization of Mannich bases with thiosemicarbazides being substituted by different cyclic and aromatic amines. The structures of the compounds were elucidated by elemental analyses, UV, IR, (1)H and (13)C NMR and ESI-MS spectral data. The in vitro antiamoebic activity was evaluated against Entamoeba histolytica in comparison with metronidazole used as reference substance. Out of the 30 compounds screened for antiamoebic activity, 10 (5, 6, 15, 18, 25-30) were found to be better inhibitors of E. histolytica since they showed lesser IC(50) values than metronidazole. The preliminary results indicated that the presence of 3-chloro or 3-bromo substituent on the phenyl ring at position 3 of the pyrazoline ring enhanced the antiamoebic activity as compared to unsubstituted phenyl ring. The study suggests that the preliminary activity of these compounds may further be explored for the development of new targets for amoebiasis.