The Cs-137 an Cs-134 monthly deposition at Tsukuba through August 1987 are reported and the origins of caesium isotopes in spring 1987 are discussed. The monthly Cs-137 deposition increased to 131 Bq m-2 in May 1986 just after the reactor accident at Chernobyl, and thereafter it reached a minimum at the end of 1986. Cs-134, which is difficult to identify in the fallout from atmospheric nuclear weapon tests, has been detected in all fallout samples since May 1986, and it shows a trend similar to the Cs-137 deposition. The caesium isotopes fallout shows a maximum in spring 1987 due to the stratospheric fallout. In 1987, about 80 precent of the Cs-137 fallout came from Chernobyl radioactivity transported into the stratosphere, and 20 percent came from previous atmospheric nuclear tests. It is estimated that several times 10-3 of released caesium isotopes from the Chernobyl reactor have been transported into the stratosphere. ¿ American Geophysical Union 1988