The blue-green sharpshooter (BGSS), Graphocephala atropunctata (Signoret), is an efficient vector of Xylella fastidiosa Wells et al, the causal bacterium of Pierce's disease of grapevine. In experiments to assess changes over time in vector transmission efficiency and populations of bacteria in BGSS, 81 of 120 BGSS that were fed for 12 h on infected grapevine acquired the bacterium. Individual infective BGSS were exposed 4 times (1 h, and 7, 21, and 60 days after acquisition) to uninfected grapevines for 3-h inoculation feeding periods to determine the efficiency of transmission to grapevine. After each inoculation efficiency test, some BGSS were tested by culturing for X. fastidiosa to determine the bacterial population in the vector heads and bodies. Transmission efficiency increased from 56 to 99% between 1 h and 7 days post-acquisition, and remained high thereafter (91% at 21 days and 76% at 60 days). Bacterial population size in the heads of the inoculative vectors also increased between 1 h and 7 days, but did not consistently increase further. Xylella fastidiosa was not recovered by culture from about 40% of the inoculative insects. Populations below the detection limit (100 bacteria) in the BGSS heads were enough for efficient transmission.