Most metagenomic analysis methods are based on the relative abundance of a marker gene or of sets of genes in functional pathways. However, these methods offer a static view of microbial composition and provide no information on growth dynamics. Korem et al. devised a new approach that measures the number of copies of a sequence at each genomic site for each species in a metagenome. For most bacterial species, a single peak and a single trough are produced, corresponding to the replication fork and replication terminus; the ratio of the two is a proxy for growth rate, as confirmed by a wide range of in vitro and in vivo measurements. Remarkably, application of the method to human faecal datasets revealed new links between bacterial growth rates, rather than composition, and type 2 diabetes, Crohn disease and ulcerative colitis.