The persistance of Escherichia coli in the larval, pupal and adult stages of both house flies, Musca domestica (L.), and stable flies, Stomoxys calcitrans (L). was examined. Abundance of E.coli declined over time in immature house flies, but remained constant in immature stable flies, suggesting house fly larvae digest E. coli but stable fly larvae do not. Survival of house fly and stable fly larvae averaged 62% and 25% respectively when reared on pure E. coli cultures. E. coli load in pupae decreased significantly one day before emergence of adult house flies, but remained constant until stable fly emergence. Nevertheless, E. coli was detected in 78% of emerging house flies and in 28% of emerging stable flies. House flies are more important E. coli vectors as adults, whereas stable flies may be overlooked vectors of E. coli during immature development. / ix, 89 leaves ; 28 cm.