The human nebulin gene includes 183 exons and four regions of alternative splicing. The mouse nebulin gene, with 166 exons, has a similar organization. Here we describe the expression patterns of one of the alternatively spliced regions of nebulin: exons 127 and 128 in the mouse gene, corresponding to human nebulin exons 143 and 144. Expression was elucidated by quantifying the differentially spliced transcripts in mice of different ages. In most of the muscles studied, transcripts expressing exon 127 were more prominent in muscles from younger mice, while older mice showed higher quantities of the transcript expressing exon 128. Some muscles, e.g., diaphragm and masseter, almost exclusively expressed only one of the two transcripts, whereas others, e.g., soleus and cardiac muscle, expressed equal quantities of both transcripts. The expression patterns did not correlate with fiber-type composition. We speculate that these exons harbor a regulatory function utilized during muscle maturation.