The U.S. charter school movement has expanded rapidly, but this growth is geographically uneven at the school district level. Focusing on Arizona--which has the least restrictive charter school law in the United States--we use district variables to determine the factors driving charter market share in 41 districts. Included in our analysis is an explicit measure of teacher dissatisfaction, which is not found in past work. Like previous analyses, we find that greater private school penetration prompts charter formation. In addition, we find quantitative confirmation of the fieldwork of Hess, Maranto, & Milliman (2001) and others suggesting that dissatisfied district school teachers drive Arizona charter growth by starting and staffing those schools. (Contains 2 tables.)