There are particular points in the services where we are invited to join the communal recitation of the prayers. Barechu, which is recited after the kaddish of Pesukei D’zimra, is one such point. The shaliach tzibbur summons everyone in the shul to join him in the recitation of the blessings of Shema, the Shema, and Shemoneh Esrei.
The opening words of Kedusha function in the same way. The shaliach tzibbur introduces the Kedusha prayer and summons everyone to join him. For this reason, the Shulchan Aruch rules that the opening words should be said only by the shaliach tzibbur and not by the rest of the congregation (125,1).
The general custom, however, is to recite the entire Kedusha. Nevertheless, the phrases beginning with “Kadosh” and “Baruch” are the primary parts of Kedusha. Therefore, if a person is reciting Elokai Natzor at the conclusion of Shemoneh Esrei and he hears Kedusha, he should interrupt his recitation and respond only to these two phrases (Mishna Berura 125: 1-2).
If he is in the middle of Shemoneh Esrei, he may not answer Kedusha. Instead, he should listen to the recitation by the shaliach tzibbur of these two lines of Kedusha. Even though it is forbidden for him to join, he nevertheless fulfills the mitzva of kiddush Hashem, publicly sanctifying G-d’s Name, by listening to these two lines of Kedusha (Shulchan Aruch 104,7).
Text Copyright © 2011 by Rabbi Daniel Travis and Torah.org