The acid unfolding pathway of apominimyoglobin (apo-mini-Mb), a 108-aa fragment (aa 32-139) of horse heart apomyoglobin has been studied by means of circular dichroism, in comparison with the native apoprotein. Similar to sperm whale apomyoglobin [Hughson, F. M., Wright, P. E. & Baldwin, R. L. (1990) Science 249, 1544-1548], a partly folded intermediate (alpha-helical content approximately 35%) is populated at pH 4.2 for horse heart apomyoglobin. For this intermediate, Hughson et al. proposed a structural model with a compact subdomain involving tertiary interactions between the folded A, G, and H helices, with the remainder of the protein essentially unfolded. As described in this paper, a folding intermediate with an alpha-helical content of approximately 33% is populated at pH 4.3-5.0 also in apo-mini-Mb. The acid unfolding pathway is similarly affected in both the native and the mini apoprotein by 15% trifluoroethanol, a helix-stabilizing compound. Thus, the folding of the apo-mini-Mb intermediate is similar to that observed for the native apoprotein, in spite of the absence in the miniprotein of the A helix and of a large part of the H helix, which are crucial for the stability of apo-Mb intermediate. Our results suggest that acquisition of a folded state in apo-mini-Mb occurs through an alternative pathway, which may or may not be shared also by apo-Mb.