Flow cytometry is a potentially valuable analytical method in microbiology providing the ability to analyze rapidly large numbers of individual microorganisms by several parameters. With a flow cytometer with enhanced light scatter sensitivity and a conventionally configured sorting cytometer, a series of comparative studies to determine the ability of the two flow systems and the antibody-direct epifluorescent filter technique (Ab-DEFT) to detect and enumerate Escherichia coli O157:H7 were made. Initial experiments used culture-derived mixtures of non-pathogenic E. coli and serial dilutions of E. coli O157:H7. Subsequent studies involved analysis of enrichment cultures from ground beef inoculated with E. coli O157:H7. Comparison of flow cytometry with microscopy and plate counts produced similar results at higher concentrations in both culture mixtures and beef enrichments. At the lowest concentrations Ab-DEFT was more sensitive, however, the time required for analysis was much less with flow cytometry. With a cytometer with enhanced light scatter sensitivity designed for bacterial analysis, O157:H7 could be distinguished from E. coli strain HB101 on the basis of light scatter. This instrument also provided direct count data for selected populations. In experiments using cell sorting to isolate target organisms, the purity of fluorescent-labeled E. coli O157:H7 sorted from beef enrichment cultures and plated was not affected by the level of background organisms, as is often the case in conventional plating procedures.