Feedlot cattle were monitored during fattening to determine changes in faecal shedding of Shiga toxin-producing Escherichia coli (STEC) and their relation to the coliform population. Faecal samples were enriched, screened for Shiga toxin genes (stx) by a polymerase chain reaction test and isolated using colony hybridization. During 117 d in the feedlot, there were differences in the numbers of coliforms shed and in the percentage of samples positive for stx. These fluctuations did not appear to be consistently related to changes in feed or time in the feedlot. The mean log coliform count for stx-positive samples (log 5.85 g-1) was similar to that for stx-negative samples (log 6.00 g-1). The STEC isolates obtained from the first 5 d in the feedlot belonged to eight serotypes. Later, one serotype (O136:H16) became the predominant STEC which appeared to be one clone as characterized by virulence determinants and pulsed-field gel electrophoresis.