A new series of bifunctional organoiron thio‐ and seleno‐terephthalate complexes — (η‐C5H5)Fe(CO)2ECO(C6H4)COX [E = S; X = C6H11NH, (C2H5)2N; and E = Se; X = PCH3C6H4NH, C6H5C2N2OS, mNO2C6H4CHCHCOO] — has been synthesized via the organic transformation reactions of the terephthaloyl chloride precursors η‐(C5H5)Fe(CO)2ECO(C6H4)COCl with the desired nucleophiles. These new complexes were characterized by elemental analysis, IR and 1H NMR spectra. The above complexes, in addition to some other selected analogues, were tested for their antifungal, antibacterial and mutagenic activity. Our results show that all the selenium‐containing compounds have antifungal activity on Candida albicans and antibacterial effects against Bacillus subtilis and Staphylococcus aureus. Four of the six selenium‐containing derivatives exhibited growth inhibitory effects against Pseudomonas aeruginosa and/or Escherichia coli. Sulfur‐containing derivatives elicited activity against C. albicans, and each one of them showed activity against at least one of the bacterial strains that have been used in this investigation. Two selenium‐ and two sulfur‐containing derivatives showed mutagenic activity against one or more than one strain of the Salmonella typhimurium using the Ames test. Copyright © 2001 John Wiley & Sons, Ltd.