The in vivo coordination structure of lanthanum ions interacting with chlorophyll-a of the fern Dicranopteris dichotoma grown in a rare earth minefield in southern China was determined by the extended x-ray absorption fine structure (EXAFS). The results show that lanthanum includes two porphyrin rings in its coordination sphere. It is postulated that the La-chlorophyll-a complex may have a bilayer structure. The analytical method may serve as a new tool to gain insight in the in vivo interactions of rare earth elements.