A peptide containing the globular region of the histone H1 from the fruit fly Ceratitis capitata has been isolated after limited tryptic digestion of insect H1. The composition of this trypsin-resistant core resembles that of the homologous peptide from calf thymus H1, although the insect H1 core possesses one cysteine, two tyrosines, one histidine, and more isoleucine and less glycine and leucine than the calf thymus H1 core. Circular dichroism measurements indicate that all the fragments that possess an ordered secondary structure (approximately 11% in both calf thymus H1 and Ceratitis H1) are present in the trypsin-resistant cores. Both calf thymus and Ceratitis H1 and their trypsin-resistant cores fold cooperatively on titration with NaOH, though the folding of the cores is less cooperative than that for the parentmolecules. On the other hand, salt-induced folding of both cores and intact molecules is noncooperative. The environment of the tyrosyl residues in both calf thymus and Ceratitis H1 has been studied by circular dichroism in the region 250-300 nm and by difference spectroscopy; their pKa' values have also been determined. The results suggest that one of the tyrosyl residues of Ceratitis H1 is buried in the hydrophobic core, in an environment similar to that of calf thymus tyrosine-72, while the second tyrosyl residue of the insect H1 molecule, which titrates with a lower pKa' value (approximately 9.30 in the absence of salt and approximately 9.80 in the presence of 0.3 M KF), is on the surface of the trypsin-resistant core. Due to the limited number of aromatic residues in the histone molecules, the above-mentioned techniques proved to be useful tools to study conformational transitions.