The chronic loss of eelgrass Zostera japonica Aschers et Graebn. has been a problem in the last decade in brackish lagoon Nakaumi, and re-establishment programs are strongly desired at present. In order to encourage the programs by offering the useful information for the eelgrass re-establishment, we conducted fi eld surveys and experiments to（1）detect inhibiting factor of seed production in the lagoon, and（2）develop preferred transplant methodology. The inhibition of seed production was detected to be caused partly by sediment perturbation, since reproductive shoot production was inhibited in the sediment perturbed treatment. Since seed production was hardly observed at the recent lagoon, we developed the preferred transplant technique using sprigs, that are sections of rhizome with shoots and leaves attatched. In the attempt, it was suggested that adequate temperature is a key for successful transplantation. Finally, we discussed the genetic problems accompanied with transplantation, in particular, “founder effect”.