To date, no definitive etiology has been described for Poult Enteritis and Mortality Syndrome (PEMS). However, two atypical Escherichia coli colony types are isolated consistently from moribund and dead poults afflicted with PEMS. To test the infectivity of these E. coli strains, poults were placed into floor pens in three isolation treatment rooms: 1) CONTROL: no bacterial challenge, 2) E. coli colony Types 1 or 2 posthatch oral challenge: 10(8) cfu/per poult at 1 d, and 3) E. coli colony Types 1 or 2 posthatch oral challenge: 10(8) cfu/per poult at 6 d. Daily intramuscular injections of cyclophosphamide (100 micrograms per poult) from 1 to 5 d posthatch were given to half of the poults in each treatment. Atypical E. coli challenge caused BW depression, and cyclophosphamide treatment exacerbated the response. All E. coli-challenged poults developed diarrhea similar to PEMS. Mortality was increased by both atypical E. coli colony types, but at 21 d E. coli colony Type 2 caused greater mortality than colony Type 1. With cyclophosphamide treatment, mortality was exacerbated with both colony types, but colony Type 2 at 1 d caused the greatest mortality. Ultrastructural damage to ileum epithelium cell microvilli and subcellular organelles indicated that part of the BW depression could be attributed to malabsorption of nutrients. It was concluded that the atypical E. coli colony Types 1 and 2 play a significant role in the PEMS disease.