One-third of 57 strains of pneumococci isolated from patients with severe pneumococcal infections in Port Moresby, Papua New Guinea, in 1978 were penicillin-insensitive (PR). The infections were community-acquired and comprised cases of bacteraemia, bacteraemic pneumonia, and meningitis. The prevalence of invasive PR strains in children was significantly greater than in adults. Twenty-one serotypes were identified, of which ten types (6, 9, 10, 12-14, 18, 19, 23, 24) showed diminished sensitivity to penicillin; with types 12 and 18 penicillin-insensitivity has not been met with previously in Papua New Guinea and only rarely elsewhere; seven of the 19 PR strains belonged to type 6. Amongst insensitive strains, minimum inhibitory concentrations of penicillin G ranged from 0.1 to 1.0 microgram/ml (resistance ratios 5 to 50). Resistance to chloramphenicol and other antimicrobial drugs was not encountered.