Who was Ithamar?
Ithamar was Aaron's youngest son (Exo. 6:23). With Eleazar the elder he succeeded to the priestly offices vacated by the death of Nadab and Abihu, which he and Eleazar were forbidden to mourn (Exo. 28:1,40,48; Lev. 10:1,2,6,7; Num. 3:3,4; 1 Chr. 24:2).
Ithamar superintended the Gershonites who had charge of the curtains and hangings, and Merari who had charge of the pillars, Cords, and boards, on the march (Exo. 38:21; Num 4:21-33).
Eleazar's family and chief men were more numerous than those of Ithamar. The high priesthood in Eli's person (probably on account of the high qualities for which he was made judge) passed to the line of Ithamar, but reverted to Eleazar's line in Zadok, because of ABIATHAR'S (which see) share in Adonijah's rebellion; thus the prophecy against ELI (which see) was fulfilled (1 Sam. 2:31-35; 3:12-14; 1 Kings 2:26,27,35).
IBSE, "Isaac" (in the public domain) with minor edits.