Hfr strains B4 and B8 transfer the Escherichia coli chromosome in opposite directions, each transferring lac(+) as the last known marker. They were mated in concurrent crosses with the proA leu metE lys trp purE lac strain chi462. Analysis of the time of entry values for these markers showed that Hfr strain B8 transfers the whole chromosome more rapidly than does Hfr strain B4. In both crosses, the rate of transfer observed decelerates. If deceleration occurs as a function of the amount of chromosome transferred, the data are consistent with the markers examined being very accurately placed on the Taylor-Trotter map of the E. coli K-12 genome.