Hydrophobically modified chitosan derivatives containing 0.6-5.1 deoxycholic acid groups per 100 anhydroglucose units of chitosan were synthesized by an EDC-mediated coupling reaction. Self-aggregates of the chitosan derivatives in aqueous media were formed by sonication with a Probe-type sonifier. The mean diameter of self-aggregates determined by dynamic light scattering decreased slightly with an increasing degree of substitution (DS) by hydrophobic groups. Photophysical and photochemical characteristics of self-aggregates were examined by fluorescence probe techniques. Measurement of the binding equilibrium constant (K-v), for pyrene and lifetime (tau) for 1,6-diphenyl-1,3,5-hexatriene (BPH) showed that the interior of self-aggregates became nonpolar as the DS was increased, Microviscosity of the interior of self-aggregates determined by measuring an anisotropy value for DPH was not significantly affected by the DS of hydrophobic groups. The aggregation number of deoxycholic acid groups per one hydrophobic microdomain !(n(DCA)) was estimated by the steady-state fluorescence quenching method with 1-dodecylpyridinium chloride and was almost independent-of the DS. From the re suits of the fluorescence quenching experiment, it was conjectured that there may be multiple (rather than one) hydrophobic microdomains in a self-aggregate.