To identify domains in the dynein heavy chain (Dhc) required for the assembly of an inner arm dynein, we characterized a new motility mutant (ida2-6) obtained by insertional mutagenesis. ida2-6 axonemes lack the polypeptides associated with the I1 inner arm complex. Recovery of genomic DNA flanking the mutation indicates that the defects are caused by plasmid insertion into the Dhc10 transcription unit, which encodes the 1β Dhc of the I1 complex. Transformation with Dhc10 constructs encoding <20% of the Dhc can partially rescue the motility defects by reassembly of an I1 complex containing an N-terminal 1β Dhc fragment and a full-length 1α Dhc. Electron microscopic analysis reveals the location of the missing 1β Dhc motor domain within the axoneme structure. These observations, together with recent studies on the 1α Dhc, identify a Dhc domain required for complex assembly and further demonstrate that the intermediate and light chains are associated with the stem regions of the Dhcs in a distinct structural location. The positioning of these subunits within the I1 structure has significant implications for the pathways that target the assembly of the I1 complex into the axoneme and modify the activity of the I1 dynein during flagellar motility.