Northern and southern Ixodes scapularis Say populations differ greatly in density, host utilization, and especially questing behavior of the immatures. Haplotypes of I. scapularis in North America can be divided into two major clades-the All American Clade (haplotypes A through J) and the Southern Clade (M through O). This genetic variation may affect feeding success and vector competence. This study compared feeding success of larval I. scapularis measured by time-to-drop-off and subsequent transmissibility success of Borrelia burgdorferi to mice using ticks from Mississippi, Connecticut (both F haplotype), and Louisiana (haplotype O). Northern ticks (CT) fed to repletion much faster than MS and LA ticks: overall, 73.6% of CT ticks had dropped off mice at Day 3 compared to only 1.7% and 6.6% of ticks dropped off for MS and LA ticks at that same time point. As for vector competence, 4 of the 4 mice in each case (MS or CT) that had been fed on by infected nymphs tested positive for B. burgdorferi. In a second experiment, 5 of the 6 mice tested positive for B. burgdorferi after exposure to infected LA ticks as compared with 3 of the 4 mice exposed to infected CT ticks. These data demonstrate that there is no difference in northern and southern populations of I. scapularis in their ability to transmit B. burgdorferi, but the ability of the northern populations to feed rapidly on rodents exceeds that of southern populations.