We measured the time-resolved fluorescence anisotropy of N-octadecylrhodamine B (C18RB) at a toluene-water interface under the total internal reflection conditions (an incident angle of 70 degrees), in the absence and presence of surfactants (Triton X-100 (TX-100), sodium dodecyl sulfate (SDS), or dihexadecyl hydrogen phosphate (DHP, pK(a) approximate to 2)). Without surfactants, the in-plane fluorescence anisotropy, r(Phi), was almost 0 independent of time, and this implied that C18RB molecules rotated very fast (rotational correlation time, tau(Phi) less than or equal to 1 ns) at the interface. In the nonionic TX-100 system, C18RB molecules rotated fast (tau(Phi) less than or equal to 1 ns), but their rotational regions were restricted by the surfactant molecules. In the cases of SDS and deprotonated DHP systems, the interfacial concentration of the protonated C18RB, C18RBH+, increased with an increase in the surfactant concentration (1.0 x 10(-10) 5.0 x 10(-8) M) by electrostatic attractive force. In these anionic surfactant systems, r(Phi), decreased exponentially with time and tau(Phi) could be obtained. tau(Phi) increased with an increase in the SDS or DHP concentration. These results suggested that the ionic interaction and frictional interaction between C18RBH+ and anionic surfactants interfered the In-plane rotation of C18- RBH+. From the tau(Phi), values, the interfacial viscosities were evaluated. The.out-of-plane fluorescence anisotropy, r(Theta), showed little dependence on the kinds and concentrations of surfactant-that is, r(Theta) values were about -0.23 +/- 0.03 independent of time. This would mean that the xanthene ring of C18RBH+ was tilted by 65 degrees +/- 2 degrees from the normal to the interface.