A total of 358 Shigella dysenteriae strains isolated from patients attending the Dhaka treatment center of the International Centre for Diarrheal Disease Research, Bangladesh, between the years 1999 and 2002 were included in this study. S. dysenteriae type 1, the dominant serotype in 1999 (76.4%), declined to 6.5% in 2002. On the other hand, S. dysenteriae types 2 to 12 were isolated with increasing frequencies of 19, 67, 73.5, and 87% in 1999, 2000, 2001, and 2002, respectively. Of these, types 2 and 4 were the most dominant serotypes, accounting for more than 18.7 and 28.5% of the total isolates, respectively. There was no isolation of serotypes 5, 7, 8, and 13 during this period. Twenty-eight (7.8%) of the isolates were atypical and agglutinated only with the polyvalent antiserum of S. dysenteriae. More than 98% of type 1 strains isolated between 1999 and 2001 were resistant to ampicillin, sulfamethoxazole-trimethoprim, and nalidixic acid. Among other serotypes of S. dysenteriae, Nal r type 2 strains were isolated in 2001 and 2002. Although heterogeneous plasmid profiles were obtained depending on the presence or absence of a single plasmid, core plasmids were defined for particular serotypes. On the other hand, the same plasmid profile was found to be shared by different serotypes. Interestingly, plasmid patterns of types 2 and 4 were almost identical except that a middle-range plasmid of 70 to 60 MDa was present in type 4 in addition to the core plasmids. All the strains harboring the 140-MDa plasmid were positive for the ipaH gene, had Congo red binding abilities, and were positive by the Sereny test, demonstrating their invasive properties. Shigellosis is one of the major diarrheal diseases afflicting humans in the developing and underdeveloped parts of the world, especially Bangladesh. Shigellosis produces inflammatory reactions and ulceration on the intestinal epithelium followed by bloody diarrhea and mucus in the stool. Shigellosis is endemic in Bangladesh, and it is estimated that dysentery accounts for 20% of deaths related to diarrhea among children (32). The genus Shigella, the causative agent of shigellosis, is comprised of four species, namely, S. flexneri, S. dysenteriae, S. boydii, and S. sonnei, and each of these species is further classified into 15 (including subtypes), 13, 18, and 1 serotypes, respectively, based on the O antigen component of lipopolysaccharide present on the outer membrane of the cell wall. S. flexneri and S. dysenteriae are the major concerns for developing countries. The manifestation of S. dysenteriae type 1 infection is more severe because of its capacity to produce Shiga toxin, which is exclusively produced by this type. Clinical infection can be transmitted by as few as 10 Shigella organisms (3), even without neutralization of gastric acid. At least three periods of epidemic outbreaks of dysentery due to S. dysenteriae 1 have been recorded previously in the Indian subcontinent, in 1972-1973, 1983-1984, and 1993-1994 (1, 9, 18, 24). Despite improvement of municipal water supplies and sanitation, shigellosis still occurs frequently. This raises important questions about the causes of shigellosis, its transmission, epidemiology, and the effectiveness of public health measures in overcoming this illness. Indiscriminate use of antibiotics in this region has resulted in the Shigella strains becoming resistant to multiple antibiotics. At present, most of the Shigella strains isolated from patients are resistant to ampicillin (AMP), sulfamethoxazole-trimethoprim (SXT), and nalidixic acid (2, 9, 15). The prevalence of the serotypes of S. flexneri in Bangladesh was described in a previous report (27), which showed that there was a temporal variation in the dominance of different subserotypes. The emergence of some atypical serotypes of S. flexneri has also been reported (27). There are no reports of outbreaks caused by the serotypes of S. dysenteriae other than serotype 1 in Bangladesh or any other part of the world. In the present study, a detailed epidemiological study of the prevalence of different S. dysenteriae serotypes and their susceptibility to commonly used antibiotics among hospital patients was conducted over the last 4 years to evaluate the present status of shigellosis caused by S. dysenteriae serotypes in Bangladesh.