The objective of this study was to examine the relationship of the relative abundance of transcripts of myosin heavy chain (MyHC) isoforms and muscling in 3 pure breeds (Pietrain, Duroc, and Mongcai) and 2 crosses [Duroc × Pietrain (DUPI) and Duroc × Berlin Miniature pigs (DUMI)]. The results of this study are that MyHC IIb fibers are the most prominent in pigs having large LM area and implies that MyHC IIb is the determining fiber contributing to the differentiation of large and small loin eye muscle area in the pig.