Sensitive yet rapid methods for detection of rare earth elements (REEs), including lanthanides (Lns), would facilitate mining and recycling of these elements. Here we report a highly selective, genetically encoded fluorescent sensor for Lns, LaMP1, based on the recently characterized protein, lanmodulin. LaMP1 displays a 7-fold ratiometric response to all Lnlus, with apparent K(d)s of 10-50 pM but only weak response to other common divalent and trivalent metal ions. We use LaMP1 to demonstrate for the first time that a Ln-utilizing bacterium, Methylobacterium extorquens, selectively transports early Lns (La-III-Nd-III) into its cytosol, a surprising observation as the only Ln-proteins identified to date are periplasmic. Finally, we apply LaMP1 to suggest the existence of a LnIll uptake system utilizing a secreted metal chelator, akin to siderophore-mediated Fern acquisition. LaMP1 not only sheds light on Ln biology but also may be a useful technology for detecting and quantifying REEs in environmental and industrial samples.