The titer of chrysanthemum yellows phytoplasma (CYP, "Candidatus Phytoplasma asteris") in the three vector species Euscelis incisus Kirschbaum, Euscelidius variegatus Kirschbaum, and Macrosteles quadripunctulatus Kirschbaum (Homoptera: Cicadellidae) was measured after controlled acquisition from infected Chrysanthemum carinatum (Schousboe) (daisy) plants. Phytoplasma DNA was quantified in relation to insect DNA (genome units [GU] of phytoplasma DNA per ng of insect DNA) by using a quantitative real-time polymerase chain reaction (PCR) procedure. The increase in phytoplasma titer recorded in hoppers after they were transferred to plants that were nonhosts for CYP provides definitive evidence for phytoplasma multiplication in leafhoppers. CYP multiplication over time in M. quadripunctulatus was much faster than in E. incisus and E. variegatus. CYP titer was also highest in M. quadripunctulatus, and this was reflected in the latent period in the insect. The mean latent period of CYP in M. quadripunctulatus was 18 d versus 30 d in E. variegatus. M. quadripunctulatus was the most efficient vector, giving 100% transmission for single insects compared with 75-82% for E. incisus or E. variegatus, respectively. By sequential transmission, we analyzed the time course of transmission: E. variegatus were persistently infective for life or until shortly before death. Occasionally, leafhoppers failed to maintain continuity of infectivity even after completion of the latent period. PCR analysis of transmitter and nontransmitter E. variegatus adults showed that some nontransmitters were CYP positive, whereas others were CYP negative. These findings suggest that both midgut and salivary gland barriers play a role in transmission efficiency.