The results of radioactivity measurements in the air of Prague after the Chernobyl nuclear power plant accident are summarized for the period of April 29 till May 10, 1986. Three separate periods of the transfer of contaminated air masses through the territory of Czechoslovakia, differing both in total activity level as well as in individual radionuclides content were indicated. Maximum of air activity at about 210 Bq·m−3 observed in region of Prague between April 30 · and May 1 differs greatly from levels measured in neighbouring countries and is discussed in relation to the trajectory lines of contaminated air masses over the west Europe.