The beta tropomyosin gene of the chicken contains a pair of alternatively spliced mutually exclusive exons the use of which is developmentally regulated. Exon 6A is used by non muscle and undifferentiated muscle cells (myoblasts) while exon 6B is exclusively used in differentiated skeletal muscle cells. A complex array of cis acting sequence elements are involved in the regulation of this alternative splicing process. Transfection assays of quail muscle cells in culture were used to define these cis acting elements. We show that, in undifferentiated muscle cells, exon 6B is skipped as a result of a negative control on its selection while exon 6A is spliced as a default choice. We provide evidence that this negative control involves a secondary structure of the primary transcript around the 5' end of exon 6B as well as intronic sequence elements located between the branch point and the acceptor splice site of exon 6B. In differentiated muscles, both exons are accessible to the splicing machinery and the preferential use of exon 6B depends on the existence of a competition between the two exons for the selection of the flanking splice sites. In particular, we show that the donor splice site of exon 6A is a weak splice site while the branch point associated with exon 6B is a strong branch point.