Secondary chemical shift analysis has been used to characterize the unfolded state of acid-denatured c-src SH3. Even though native c-src SH3 adopts an all-beta fold, we found evidence of transient helicity in regions corresponding to native loops. In particular, residues 40-46, connecting the n-src loop to the third beta-strand, exhibited an apparent helicity of nearly 45%. Furthermore, the RT loop and the diverging turn appeared to adopt non-native-like helical conformations. Interestingly, none of the residues found in transient helical conformations exhibited significant varphi-values [Riddle, D. S., et al. (1999) Nat. Struct. Biol. 6, 1016-1024]. This indicated that the transient helicity has no influence or only a weak influence on the actual protein folding reaction. The residual structural propensities were compared to those of other SH3 domains, revealing heterogeneity in the unfolded ensemble that clearly contrasts with the conserved character of the topology of native state and transition state ensembles typical for SH3 domains.