A polymerase chain reaction (PCR) assay was used to characterize 309 hemolytic E. coli isolates. The isolates were obtained from swine specimens presented to the diagnostic laboratory between August of 1996 and August of 1997. About one-half of the isolates contained genes for enterotoxin and/or Shiga toxin. Enterotoxigenic E. coli (ETEC), which cause diarrhea, were much more prevalent than Shigatoxigenic E. coli (STEC), which cause edema disease. K88 was the most common pilus type among ETEC and F18 was the only pilus type identified among STEC. These data are consistent with the notion that E. coli induced diarrheal disease is more prevalent than edema disease. However, they demonstrate that STEC persist in the swine population in spite of the low prevalence of clinical edema disease in recent years. The data suggest that vaccination and vaccine development based on K88 and F18 pilus antigens continue to be relevant for hemolytic E. coli infections. Some of the isolates that did not have genes for either enterotoxin or Shiga toxin, had genes for K88 or F18 pili. Such nontoxigenic isolates (NTEC) are probably not pathogenic and were speculated to act as naturally occurring K88 and F18 vaccines in some herds.