Aminomethylation (i.e. Mannich reaction) of benzimidazole was carried out by treating benzimidalzole with formaldehyde and 4-aminosalicylic acid. The resultant compound was designated as 1-(4-carboxy-3-hydroxyphenyl aminomethyl) benzimidazole (BI-SA). The transition metal complexes of Cu2+, Co2+, Ni2+, Mn2+, Zn2+ and Fe3+ of BI-SA have been prepared and characterized by elemental analyses, spectral studies, magnetic moment determination, molar conductivity measurement and microbicidal activity.