Synthesis of some new 3,5-diamino-4-(40 -fluorophenylazo)-1-aryl/heteroarylpyrazoles (5a–5i) was achieved by the treatment of aryl/heteroarylhydrazines 4a–4i with 2-[(4-fluorophenyl)hydrazono]malononitrile 3 in refluxing ethanol. The structure of the compounds was established on the basis of IR, NMR (1 H and 13C) and mass spectral studies. All the nine synthesized compounds were screened for their in vitro antimicrobial activity against two Gram-positive (Staphylococcus aureus, Bacillus subtilis), two Gram-negative bacteria (Escherichia coli, Klebesiella aerogenes) and three pathogenic fungi (Aspergillus niger, Candida albicans, and Saccharomyces cerevisiae).