Arbuscular mycorrhizal (AM) fungi produce an extensive hyphal network that develops in the soil. A symbiotic relationship is formed between this network and most terrestrial plants with the fungi receiving carbon in return for nutrients. This transfer of carbon from plants to fungi is a crucial component of the terrestrial carbon cycle, but the microscopic size of the hyphae has made it difficult to determine the residence time of carbon in these fungi. In the May 16 Science, Philip Staddon and colleagues at the University of York used accelerator mass spectrometry (AMS) to show that the hyphae live for about 5 to 6 days. Their data also indicate that the transfer of carbon to the hyphal network is rapid and on a large scale (Science, 300:1138-1140, May 16, 2003).