The postnatal expression profiles of -sarcomeric actin transcripts and protein are quantified in mouse striated muscles from birth to postnatal day 56 by Northern and Western blot analyses. -Cardiac actin (-CA) transcripts transiently increase between 12 and 21 days after birth in the quadriceps muscle, reaching ~90% that found in the adult mouse heart. Although -CA is the -sarcomeric actin isoform expressed in the immature fiber, the expression profiles of other contractile protein isoforms indicate that this postnatal period is not reflective of an immature phenotype. -Skeletal actin (-SA) transcripts accumulate to ~32% of the total -sarcomeric actin transcripts in the adult heart. Our study shows that 1) there is a simultaneous reappearance of -CA and -SA in postnatal skeletal and heart muscles, respectively, and 2) the contractile protein gene expression profile characteristic of adult skeletal muscle is not achieved until after 42 days postnatal in the mouse. We propose there is a previously uncharacterized period of postnatal striated muscle maturation marked by the reappearance of the minor -sarcomeric actins.