Distribution of Ulva pertusa in an intertidal flat in Mikawa Bay was investigated visually from a boat and aerially with a multi spectral scanner from a plane in the summer of 1985. The values of standing crop and vegetation area estimated from these two methods agreed well. The growth rate of 7% per day was obtained, and was considered to be reasonable as a net growthrate obtained in situ. It was comparable to the net photosynthesis previously determined from the nitrogen budget in the flat. Therefore, it is strongly suggested that U. pertusa together with Zostera marina instead of phytoplankton is responsible for the photosynthetic activity in the flat during summer and its limiting factor is nitrogen.