Fecal samples were collected from 4,361 dairy cows on 91 dairy operations between 26 February and 8 July 1996. Fecal samples were cultured for Escherichia coli O157, and positive isolates were probed for verotoxin-producing genes. A total of 52 (1.2%) fecal samples on 22 (24.2%) operations were positive for verotoxin-producing E. coli O157. Herds in which samples were collected on or after 1 May 1996 were significantly more likely to test positive than herds sampled before that date (odds ratio = 7.7). Herds maintained on farms on which alleyways were flushed with water to remove manure were 8.0 times more likely to have samples test positive for verotoxin-producing E. coli O157 than were herds maintained on farms cleaned by use of other methods of manure removal.