SUMMARYFrom 1979 to August 1987, there have been 178 cases of meningococcal disease in Iquique, Chile, a city of about 140000. The attack rate for the last 5 years has been in excess of 20/100000 per year, more than 20 times greater than for the country overall. The mortality rate was 6 %. The disease occurred in patients with ages from 4 months to 60 years, but 89 % of cases were in patients < 21 years. The largest number of cases were in the age group 5-9 years (n = 54), but the highest incidence occurred in children less than 1 year of age (72-8/100000 per year). The male/female ratio was 1-2. Cases occurred all year round with little seasonal variation. Of the 178 cases, 173 were biologically confirmed. Serogroup analysis of strains from 135 patients revealed A = 1, B = 124, C = 10. Forty-four group B strains from 1985-7 were serotyped: 15:P1.3 = 36, 15:NT = 4, 4:P1.3 = 2, NT: NT = 2. Ten of 11 of the outbreak strains tested were sulfadiazine-resistant. This is the first recognized outbreak caused by a Gp B: 15 strain in South America. It shares many of the characteristics of outbreaks caused by closely related strains in Europe, such as a predilection for older children and adolescents, sulfadiazineresistance, and sustained high attack rates. The Iquique strain (B:15: P1.3) belongs to the same genetic clone (ET-5 complex) as the Norway (B: 15: PI. 16) and the Cuban (B :4: P1. 15) strains.