probabilistically reconstruct transmission networks (Videos S1 and S2). Calculating the average number of secondary cases per rabid dog during the period of exponential epidemic growth (before vaccinations were implemented) from these reconstructions gave similar R 0 estimates of 1.1 in Serengeti district and 1.3 in Ngorongoro (CIs: 1.04-1.10 and 1.26-1.42, respectively) ( Table 1 ). The more traditional approach of estimating R 0 , by fitting a curve to incidence data over the same interval of exponential epidemic growth, also produced similar estimates of 1.2 in Serengeti and 1.1 in Ngorongoro (CIs: 1.12-1.41 and 0.94-1.32, respectively) ( Table 1 and Figure 3B ). This approach is robust to underreporting (Text S1 and Figure S2 ) but should likewise be considered a lower bound, because some local control measures were instituted (such as tying or killing). We also estimated R 0 from the intrinsic growth rate of outbreaks of domestic dog rabies elsewhere in the world ( Table 2) and obtained values between 1.05 and 1.85, which are consistent with our estimates from northwest Tanzania.