The relationship between endemic bacteriophages infecting E. coli O157:H7 (referred to as "phage") and levels of shedding of E. coli O157:H7 by cattle was investigated in two commercial feedlots in southern Alberta, Canada. Between May and November 2007, 10 pens of cattle were monitored by collection of pooled fecal pats, water with sediment from troughs, manure slurry from the pen floor, and rectal fecal samples from individual animals (20 per pen) at two separate times. Bacteriophages infecting E. coli O157:H7 were detected more frequently (P<0.001) after 18 to 20 h enrichment than by initial screening and were recovered in 239 of 855 samples (26.5% of 411 pooled fecal pats, 23.8% of 320 fecal grab samples, 21.8% of 87 water trough samples, and 94.6% of 37 pen floor slurry samples). Overall, prevalence of phage was highest (P<0.001) in slurry. Recovery of phage from pooled fecal pats was highest (P<0.05) in May. Overall recovery did not differ (P>0.10) between fecal grab samples and pooled fecal pats. A higher prevalence of phage in fecal pats or water trough samples was associated (P<0.01) with reduced prevalence of E. coli O157:H7 in rectal fecal samples. There was a weak but significant negative correlation between isolation of phage and E. coli O157:H7 in fecal grab samples (r= -0.11; P<0.05). These data demonstrate that the prevalence of phage fluctuates in a manner similar to that described for E. coli O157:H7. Phage were more prevalent in manure slurry than other environmental sources. The likelihood of fecal shedding of E. coli O157:H7 was reduced if cattle in the pen harbored phage.