OBJECTIVE Diphtheria infections caused by the different toxigenic biotypes of Corynebacterium diphtheriae are endemic in Delhi. Information on biochemical identification, toxigenicity and antimicrobial susceptibility to this bacterium is scanty. This retrospective study was carried out to identify isolated Corynebacteria biochemically, determine their toxigenicity, drug sensitivity and some epidemiological characteristics of diphtheria cases from Delhi and adjoining States for the period 1998-2004. METHODS A total of 1118 throat and 585 nasal swabs were used to detect human pathogenic corynebacteria. WHO recommended methods were used for the detection, screening, toxigenicity and antibiogram pattern of the isolates. RESULTS Among 493 (44.1%) cases detected positive for corynebacteria 71.8 per cent were pharyngeal, 20.9 per cent nasopharyngeal and rest 7.3 per cent nasal diphtheria cases. Biochemical identification revealed two species i.e., C. diphtheriae and C. pseudodiphtheriticum. In C. diphtheriae three biotypes were detected viz., intermedius (95.5%), gravis (3.4%) and mitis (1.1%). Toxin was expressed by 96 per cent isolates of C. diphtheriae. Cases were recorded from Delhi and four adjoining States. Sex ratio among male to female was 1.6:1. Prime victims were less than 9 yr old children (93.3%). Unvaccinated children (70.2%) were the main sufferers. Fatality rate was highest in Delhi cases (16.8%) followed by UP (14.6%) and Haryana (5.9%). CONCLUSIONS Standard methods revealed the replacement of C. diphtheriae var mitis with var intermedius and occurrence of diphtheria infections due to other human pathogenic corynebacteria. It is imperative to have good bacteriological facilities to have better surveillance with regular monitoring in the endemic areas to keep the disease under control.