Escherichia coli O1157 is one of the most common vehicles of foodborne outbreaks. Sensitive detection procedures are necessary to determine the small numbers of microorganisms in the sample because the infective dose of this agent is very low. In this study, the efficiency of two different enrichment media and two different incubation periods, besides the comparison of the sensitivity of direct plating method and immunomagnetic separation (IMS) were investigated for determining the most suitable method for isolating low count E. coli O157 from raw hamburger samples. Enrichment in Buffered Peptone Water (BPW) and mBPW for either 6 or 18 hours was performed for experimentally contaminated samples with three different inoculation levels of E. coli O157. Isolation was carried out both directly after the enrichment and after IMS application. The results of this study indicated that performing IMS improved the sensitivity of the isolation procedure significantly. 1 cfu/25 g E. coli O157 can easily be isolated with this method. The enrichment procedure has an important effect on the sensitivity of the isolation. Using selective enrichment broth decreased the sensitivity compared to nonselective broth.