The W-Beijing strain of tuberculosis has been identified in many molecular epidemiological studies as being particularly prevalent. This identification has been made possible through the development of a number of genotyping technologies including spoligotyping. Highly prevalent genotypes associated with outbreaks, such as the W-Beijing strain, are implicitly regarded as fast spreading. Here we present a quantitative method to identify ''emerging'' strains, those that are spreading faster than the background rate inferred from spoligotype data. The approach uses information about the mutation process specific to spoligotypes, combined with a model of both transmission and mutation. The core principle is that if two comparable strains have the same number of isolates, then the strain with fewer inferred mutation events must have spread faster if the mutation process is common. Applying this method to four different data sets, we find not only the W-Beijing strain, but also a number of other strains, to be emerging in this sense. Importantly, the strains that are identified as emerging are not simply those with the largest number of cases. The use of this method should facilitate the targeting of individual genotypes in intervention programs. mutation ͉ transmission rate ͉ Beijing strain ͉ infectious disease ͉ molecular marker A broad goal of the development of effective tools for genotyping the bacteria or viruses causing infectious diseases has been the classification of isolates into distinct types. In the case of Mycobacterium tuberculosis, one of the outcomes of this development has been the identification of a particularly aggressive strain known as the Beijing or W-strain (1, 2). These genotypic data, however, can also be used to verify chains of transmission and to make inferences about population level transmission patterns. For example, the occurrence of large clusters of identical genotypes in a sample is thought to be indicative of recent tuberculosis transmission (3, 4), and in this context, the size of a genotype cluster carries some information about the rate of transmission associated with the genotype. One use of such information is to study possible risk factors for infection, such as HIV status, by correlating them with the extent to which these data form clusters (3, 4).An unusually large cluster in a sample may indicate a rapidly spreading strain; however, it may simply indicate the age of the genotype (5). For instance, strains that have been present in a population for a long time may have accumulated a large number of cases despite having a slow transmission rate. One way to access information about the age of a strain is to consider the number of mutation events identified in the history of that strain. That is, an old genotype has had ample time to generate many mutants, which should be manifested in the sample. We observe that on average, if two strains have the same cluster size and the same mutation rate, then the one with more observed mutation events is older, and correspondingly, be...