Retaining effective teachers is a key policy priority nationwide, particularly in districts that serve large numbers of disadvantaged students. We investigate whether a change in the Miami region's placement strategy was accompanied by changes in teacher attrition and mobility decisions. Our results suggest that the increased concentration of TFA corps members in schools was associated with a reduction in TFA mobility across schools after the first year of service, but showed no association with the overall retention of corps members in the district after the two-year commitment. We also find that TFA corps members eventually retained beyond the two-year commitment performed substantially better in mathematics during their first two years of teaching: evidence of positive selection into post-commitment retention. [This paper was published in "Educational Evaluation and Policy Analysis" v38 n3 p495-516 2016 (EJ1108469).]