Chronic wasting disease (CWD), a contagious prion disease of the deer family, 
has the potential to severely harm deer populations and disrupt ecosystems where 
deer occur in abundance. Consequently, understanding the dynamics of this 
emerging infectious disease, and particularly the dynamics of its transmission, 
has emerged as an important challenge for contemporary ecologists and wildlife 
managers. Although CWD is contagious among deer, the relative importance of 
pathways for its transmission remains unclear. We developed seven competing 
models, and then used data from two CWD outbreaks in captive mule deer and model 
selection to compare them. We found that models portraying indirect transmission 
through the environment had 3.8 times more support in the data than models 
representing transmission by direct contact between infected and susceptible 
deer. Model-averaged estimates of the basic reproductive number (R0) were 1.3 or 
greater, indicating likely local persistence of CWD in natural populations under 
conditions resembling those we studied. Our findings demonstrate the apparent 
importance of indirect, environmental transmission in CWD and the challenges 
this presents for controlling the disease.