Numbers 4:33

This [is] the service of the families of the sons of Merari,
&c.] The Mahlites and Mushites, namely, the service before observed, the particulars of which are given:

according to all their service in the tabernacle of the congregation;
agreeably to what was allotted to them in that part of the tabernacle they were concerned in:

under the hand of Ithamar the son of Aaron the priest;
who had the oversight of these as well as of the Gershonites, as Eleazar had of the Kohathites; though as Eleazar was the chief of the tribe of Levi, it is thought that Ithamar acted under him, ( Numbers 3:32 ) .