Femtosecond fluorescence upconversion was used to study the excited-state dynamics of two-dimensional J-aggregates of a thiacyanine dye. At high intensities the excited-state population decay is faster than exponential and the decay is intensity dependent. A singlet-singlet exciton annihilation decay mechanism is consistent with the observed behavior. A value of 4 x 10(12) molecule s(-1) is calculated for the exciton annihilation constant gamma, Exciton annihilation measurements reported in the literature for J-aggregates of several cyanine dyes are recast into the same units for comparison. The physical state of these J-aggregates is elucidated to explore the relationship between the magnitude of the exciton annihilation constants and the liquid-crystalline nature of these aqueous colloidal systems. Attempts to time-resolve spectral features related to intraband relaxation in the excited excitonic states of the thiacyanine J-aggregate prior to annihilation indicate that these relaxation processes are faster than 100 fs at room temperature.